A kind of reasonable cherry pick, except you didn't even pick it right. Seedot has an explicitly gimped movepool compared to Nuzleaf, and ORAS Nuzleaf's movepool is basically to a modern standard already. RSE Nuzleaf's movepool is a point worth making, but even then the line gets a ton of use out of a few barely contested earlygame TMs like Bullet Seed, Thief, and Rock Tomb. What does fit your thought is the fact that Nuzleaf and Shiftry do definitely fall apart towards the end of the game, and really rely on Sunny Day (or Groudon I guess) and Solar Beam to do much of anything past a certain point. But before then, stuff like (60 BP) Giga Drain, Faint Attack, Brick Break / Shadow Ball, Nature Power, and potentially a decent Hidden Power are totally fine to be using. Speaking of which,
This wouldn't prevent Vulpix from using Will O Wisp and Hex, it could still learn both of those moves. It just would also be allowed to function as a Fire type too.
On a literal level, of course it isn't true that having access to Flamethrower means you can't use Flame Burst.
But like. Would you?
It might seem somewhat bizarre to consider this happening with a strong STAB and a coverage move but think about it. The use cases of your second strongest attack are only as common as the situations where your first strongest attack is weaker than it. For situations where moves are differentiated by type, that means situations where the strongest move is NVE or your move is SE. And for you to be using a pokemon against another, it's probably going to be one of the better options on your team at handling a threat- we're talking ingame here, so there's no need to worry about reads, switch-ins, or mid-ground plays. The amount of situations where a pokemon is going to need to use its third or even second strongest attack in a given playthrough is
low, much lower than you might expect. If you have let's say Flame Burst on Vulpix, you probably just aren't using Hex very often - you'd probably have another teammate handle Rock / Ground / Water types, (if you aren't just using Energy Ball or Solar Beam for them instead) and in Fire mirrors the combo around it doesn't work. So you're only using it for Psychic, Ghost, and Dragon type matchups, and even that is assuming you don't have a better way to deal with any of those with a different pokemon on your team.
With the whole thing about preforming the role of a type, I also don't agree. Having the utility of offensive and defensive typings in pokemon spliced apart from one another is not bad design for ingame at all imo. There is something you can get that's different from old gen movepools (occasionally barren to the point of uselessness, filled out with random utility-less normal moves, excessively reliant on staple TMs / HMs) but that isn't just as cookie cutter as some modern movepools.
