And later you use it to lose Omg you failed your mission 100%

Why would you spend potential hours grinding up another water type when you can have a high level water starter as default? Grass and Fire aren't valuable types for a team, unlike water, so not only can you replace those with other alternatives too, but you can even leave them out completely. A water type is insanely good though, but often the ones you can get in a playthrough are massively underleveled, and you're handicapping yourself hard by taking one instead of just using the water starter.

Ironically it's the other way round: In professional runs Feraligatr is better in gen 2, Typhlosion is better in gen 4. There are two main reasons. The first is Typhlosion's lack of actual fire moves in gen 2. It gets Flamethrower on level 60, which essentially means the move is generally unavailable except for Red unless you want to do a massive grind. Fire Blast is unaffordable until the very end too and grinding the game corner for an early Fire Blast takes longer than actually beating the game. Even Fire Punch is learned way too late as only Typhlosion gets it, leaving it with Flame Wheel until level 36. Thunderpunch is also only available at that level, leaving Quilava pretty helpless for the water route and the 5th badge that are in the level 30 area. Meanwhile Feraligatr gets Surf the moment you enter Ecruteak City, giving you a high power water move extremely early. With how much stronger Surf is, Feraligatr will outdamage Typhlosion for essentially the entire game up to Red, especially with the already mentioned stat exp system. The other main reason is the one hole in Typhlosion's fire/electric coverage: dragons. With the final gym and the champ being dragon trainers that often also use water moves, they are massive roadblocks. Roadblocks that Feraligatr doesn't have thanks to Ice Punch. The only holes in Feraligatr's coverage are other water types, but with its high atk it can muscle through with normal coverage. In the remakes Goldenrod was massively buffed and now sells Fire Blast and Hyper Beam, allowing Quilava to just straight up oneshot Whitney's Miltank from full. You also get a free choice specs at lake of rage, giving you truly absurd levels of power. Utilizing battle style switch and your hm users as fodder you can work around Hyper Beam's recharge turn and coupled with Choice Specs this finally gives you the raw power to oneshot dragons with Typhlosion, removing its only roadblocks.