The original Mr. X made RE2 actively worse, because optional bosses (especially ones that inexplicably drop ammo as a reward) don't mesh well with the kind of game the classic REs tried to be. Original Mr. X and Nemesis are at their core nothing more than optional boss fights. They're easy to flee from and they dispense rewards if killed. The problem is that the game has to assume you'll fight them and give out correspondingly large amounts of resources, which you can then just pocket by running by the bosses. Not coincidentally, RE2 and RE3 are generally seen as the easiest entries in the classic series (any difficulty in RE3 comes from being designed around that awkward dodge mechanic).
Nemesis and Mr. X are very different both when it comes to fighting them and running away from them. Nemesis is much harder to avoid and infinitely harder to take down. Also, I think your logic that you have more ammo because of him is flawed. Yes, you could argue the game has to assume you are going to fight Nemesis and give you more ammo, but that's true even without him. In all classic Resident Evil (and RE2make) you have way more ammo they you need, maybe not to kill every enemy, but certainly to take more than enough to breeze through. Ammo management in RE doesn't come from having little of it, or at least that's just part of the equation, the other part if not knowing if you have enough ammo or not. And in that context an enemy like Nemesis who takes huge amount of bullets to take down will always keep you on your toe. That's why RE games get so much easier (and faster) to beat in subsequent playthroughts, that element is gone. That's also why when we analyze this subjects we have to think of first time players, and not veterans of the game who know all their mechanics, ammo location, etc...
I'd also like to add that even if we assume combat is a weak part of the Resident Evil, which I think is not that true in RE3, ammo isn't the only resource that is important to it. You're missing the health aspect of it. Nemesis can deliver extreme damage to the player and is very hard to take down. He is not that hard to ignore, that's true, but only when you're confident where you're going. Which first timers won't be. And RE3 doesn't give you considerably more health itens just because Nemesis is out there. To finish, I'm not sure if people will say RE3 is among the easy RE games.
Incidentally, this is one of the reasons I'm not too excited by a RE3 remake - it was kind of a mediocre, clunky shootbanger in the first place and the presence of Nemesis (who presumably will function more like his original version than Remake Mr. X for obvious, sprinting-and-rocket-launcher related reasons) virtually guarantees that it'll be too action packed. I'd prefer Capcom remake Code Veronica, which was not only an extremely flawed game that could very much benefit from a remake, but also reigned in the shooter gameplay and so is much more fertile ground for a survival horror remake. Or just make originals that focus on the high points of the police station and Baker house.
I strongly disagree withy your assessment of RE3, but I can see where it's coming from. Its balance is indeed more focused on action, on killing key enemies rather than avoiding them (which is one of the reasons why it can give yo more ammo and still make you tense), on combat rather than strategy. But in the end, it still is very much a survivor horror. I do think R3make will be more action oriented than RE2, mirroring the originals in a sense, but that doesn't make it worse.