Something I debated last night in a server, it was about buffing early route mons. The other party didnt like the idea, even though I noted that changing evo level to compensate so early main game campaign wouldn't be as affected, while still helping them late game and comp. They argued that the principle of cycling weak mons should be respected, except...
Literally every game the starter is just better immediately (well bar Starly)
So theres little to no point using an early route mon regardless (I'm aware of nuzlockes, but most casuals don't do those)
Well the thing is you have to look at them in the context of how they perform throughout the game. The starter is probably "better" off the bat as well as in the long term, but the thing is that you cannot rely solely on your starter to get through the game, even early on. You will need mons to cover for whatever your mon is weak to throughout the game. So if you have the Water starter, you will need to cover for Grass-types, and the early game Bug-type and bird will do well to cover that, the former at least early on.
Let's look at a case like, say Caterpie and Weedle, for example. These mons have a distinct gimmick for an early game Pokemon: they evolve all the way through a three-stage metamorphosis phase early on. The early Bugs are a distinct tutorial mon for a new player that teaches the player several important concepts, mainly that of evolution. So let's say you catch a Caterpie and start raising it. Then you raise it to Level 7 and it evolves into Metapod, then at Level 10 it evolves once more into Butterfree. It does this well before your starter evolves even once, and through the larva-pupa-imago transformation that caterpillars go through to become butterflies, which is something many kids are taught in elementary school, so this explicitly teaches them the concept of evolution in a way that's intuitive to a kid. Once a kid sees this, they will understand what is happening when their starter reaches Level 16 and begins to evolve the first time.
And once the Bug fully evolves at Level 10, that Butterfree or Beedrill (or whatever early Bug they have) will be a powerhouse early on. Compared to all fully evolved Pokemon, yes, they stand out as notably weak, but at the point where they first evolve? They are much stronger than anything your opponents will throw at you and will be a powerhouse early on, being able to take hits from their first stage mons and hit back hard in return. Anything your starter cannot defeat, your Bug will surely help cover against them. Butterfree and Beedrill are very strong for the early portion of the game, and will continue to be a helpful crutch until, say, the mid 20s or 30s, where they then fall off. Yes, this encourages the player to rotate their team, because the game wants you to explore and catch the mons who show up later in the mid-game and late-game, and so the first five Pokemon you catch shouldn't be strong enough to cover your needs for the whole game. If it's Pidgey, Starly, Fletchling, or whatever variant of the three-stage early bird, then yes, you will probably be inclined to keep it because the early route bird is designed to be good for the whole game, but your Bug will be first in line to go out the door, and the rodent soon after.
And though the early game mons fall off as the game goes on, they aren't terrible the whole way through. Rather, they are crutches who will help deal with anything your starter cannot for the early portions, so that you always have backup for your starter at any point, so you don't have to deprive yourself of teammates for your starter until the mid and late game if the good mons don't show up until then. That early game Bug evolves all the way and teaches a player the concept of Pokemon evolution, and then serves as a strong teammate for the early portion of the game. That rodent/mammal with Raticate, Bibarel, etc. is a hard carry that is a catch all attacker against everything: Rattata for example learns Hyper Fang early, and evolves early at Level 20, so until the 30s, it's a powerhouse with a strong Hyper Fang that decimates most of the early game opponents until it falls off at around Level 30, where better Normal-types like Snorlax, Kangaskhan, and Tauros start showing up. At that point, Raticate's stats won't cut it anymore, but at the same time, your available options increase, so now you make room for more teammates to take their place. Or take a case like BW Watchog, who can be a competent team member until around, say the high 30s, at which point you gain options like Cinccino, Sawsbuck, Scrafty, and Bouffalant.
That's the thing with early game mons: you have to look at them in the context of how they perform
at the point where they are obtained. If you look at things like the Bugs and the rodents in a vacuum, compared to every other fully evolved Pokemon, yes, they stand out as notably weak. But in the context of the early portion of a typical Pokemon adventure, they are quite strong for the early portion. They grow stronger very quickly, evolve early, and their stats are strong for the point where they evolve. The Bug will carry you for the early segments of the game as it's faster and stronger than most of what your opponents will throw at you. The Rodent, who evolves at Level 20 usually, will kick butt against most of the mons who show up in the 20s. And the rodent's neutral coverage and power will make them a strong catch all attacker. They don't keep up long term, but they can cover your starter early on while you still raise your starter slowly to its epic final form, which is still a long ways away, and help against opponents your starter cannot cover early on.
Your starter is, for all intents and purposes, of course, designed to be the sole permanent member of your team aside from the bird, but it reaches its full potential relatively slowly, evolving once at Level 16, but reaching its final form in the 30s, so it won't always be equipped to handle everything early on, until it reaches its final form in the 30s. The early game mons may fall off, but they have the boon of growing strong quickly and carrying you before doing so, all the while your starter steadily grows stronger until it finally reaches its epic and awesome final form. At the point your early game mons fall off your available options increase, and with it you can get even stronger Pokemon to start building a proper team around your starter with teammates who are of a similar caliber of power to where your starter's final form is at.
One of the goals of a Pokemon game is to complete the Pokedex, and to catch every mon in the Dex, so catching the early mons, then ditching them when they fall off and replacing them to make room for new ones later on, is the main way this is designed to work: you keep catching mons and having a team that serves as your collective gear and squad for every point of the game, swapping out weaker ones for stronger ones as the power level of the game rises, ensures that you will continue to progress towards filling the Pokedex in such a manner.