I think the kanto "starter you pick is the difficulty level" idea is bullshit. If the game wanted to intentionally do it this way, it'd have to explicitly tell you it's that way. Realistically, this only affects the first two gyms only a tiny bit, because Misty has some available Pokemon you can use to deal with her if you chose Charmander (a few Electric, Grass and Bug pokemon, a few users of status, and some generally strong physical attackers like Raticate, Nidos, hell even Gyarados with enough determination). Brock is only really hard if Charmander is underleveled or not using items, otherwise Charmander or Charmeleon should have an okay time with him, and Butterfree is still an option if you must have a neutral attack. Hell, in FRLG, that isn't even necessary because Charmander learns Metal Claw and Mankey is more freely available! It's just a dumb notion
FRLG's option, or I suppose the way most games post-FRLG do it, is so much better I'm amazed it took that long to implement. All three starters can put up a decent fight against Brock in FRLG, even despite Charmander being at a defensive disadvantage.
Up until about your second or third badge, your starter outranks pretty much every other obtainable Pokemon statwise (and more so once it's evolved) so most players do have to depend on it. But this becomes a non-issue once you're 3 or 4 badges into the game, so for that initial period it makes more sense to give each starter a fighting chance against the first couple of gyms.
One thing that stands out to me is that in the little instruction manual that comes in the box for Pokemon Red & Blue, one of Oak's little tips is: "first time players should pick Bulbasaur" (image below).
Like, I get that they expected most people to read the manual. If you're not going to implement a proper difficulty level like B2W2 did, it's not the worst idea in the world to have an easy/moderate/difficult starter. But nothing in the game hints at that advantage, and it defeats the notion that you're meant to choose the one you like best.
You don't even need to make one of the starters better than the others so long as the first gym doesn't have an advantage against any of them. Just make the first gym Normal or Fighting or Dark or Psychic (or, dare I say it, Dragon). As much as I'm not fond of BW's first gym, it did put everyone on an even footing so that's something.