You mean quality over quantity? Why not both?
Having only two realistic end game crafts is rather boring. The firestorm is your answer to everything except if it is too big, then it's the avenger. It is not that long to access them either, although getting enough Elerium to operate can be an issue.
I like games where the progression is smooth with small improvements that make me feel like I have accomplished something and give me something new to entertain myself with until the next new development. If this process has too few steps, what usually happens is:
State 1 (say interceptor as your fighter craft):
1.1 - Easy
1.2 - Challenging and fun (UFOs are bigger, you get damaged, you have to think)
1.3 - Too hard (UFOs got too large quickly, interception is useless as you just get blown up)
State 2 (Firestorm comes in!)
2.1 - Game is easy (the firestorm is much better than an interceptor and you probably are getting plasma beams soon)
2.2 - Challenging (UFOs get bigger or more of them appear)
2.3 - Hard (You need swarms of Firestorms, lose some any ways)
State 3 (Avenger!!)
3.1 - Game is easy: Avenger rules the sky
... Depending on difficulty, you might reach 3.2 or 3.3 too.
I'd rather the game be challenging and fun the whole time. Setting up bases around the world means I tend to keep at least 6 fighters for interceptions (NAmerica, SAmerica, Europe, Africa, Asia, Australia). With a continuous stream of upgraded fighters, my workshop can be building the latest craft I discovered, while the continent I care about the least is still using crafts from a few generations back, so I get to use everything for a while any ways.
Furthermore, having various crafts doesn't mean that they necessarily all become obsolete. Having different crafts to intercept different UFOs (super fast one to intercept small UFOs, slow and resilient one for the big ones) gives options. What you are saying is a bit like: "I don't need lasers in the game, they get overshadowed by plasma too quickly". My reaction is: "How can I make both relevant, at different stages of the game (laser should come first) but together as well (it should never be irrelevant)".