Sure, it's Wesker I'm talking about, but now we're looking at it in retrospect. By the time you are referring to, he was nothing but "another villain". He didn't even have red eyes by that time. I doubt that the original developers of Resident Evil were already planning to bring him back, and I never even thought of it after finishing the game for the first time.
Imagine he would have stayed dead and Irons would have returned instead. No one would be talking about Wesker anymore.
Imagine a Resident Evil 10 that features Piers (who is a well-established main character by then) and a new partner who dies at the end. Someone will claim: "He'll be back! After all, Piers came back too, didn't he?" And someone else will answer: "Yeah, but it's Piers we're talking about." You get my point? As Jenevieva said, we don't know what the developers are planning for those characters (and I'm even sure they don't really know it themselves, otherwise they wouldn't have to do so many remakes to set things right).
Then let us sound stupid together, because I can't handle bad endings for good characters either. Actually I don't really care if we ever get to play Piers again, if only I know he's alive and well. (I would still appreciate it, though.)