Sure, any great bassist could reproduce the old songs on stage
But they are like the only (metal) band where the bass is actually more prominent than guitars
Since they are much more melody- than riff-oriented, the majority of rhythm and texture are provided by bass
During the verses, the guitars often just mimic what bass is doing (NOT vice versa!
)
Sometimes the bass carries the melody while guitars just play the chords (To Tame A Land for instance)
Sometimes the song IS riff-based, but bass has its own life and interplay with the riff (like SIASL, 2MTM)
You can clearly hear the songs are written by the bass player. In fact, the songs credited to others (like Revelations, Powerslave, 2MTM, even Gangland) have GREAT bass lines.
One more aspect which I'm not sure how many people noticed, but his bass playing is actually quite "sloppy", he is all over the place rhythmically. Just listen to this:
And while that is generally a "bad" trait for bass, it fits Maiden like a glove, and helps increase the feel of playfulness (as opposed to heaviness of having the bass strictly on time), especially in interaction with the same kind of drummer (Nicko more than Clive), playfulness that is this band's real forte
I think this alone would deem him "irreplaceable", as far as bass playing alone is concerned
Now I admit that lately (reunion) he's probably been more concerned with other things on albums (production?), because his playing is a bit understated, that's why I was delighted when I first heard Book of Souls, I said immediately "YES! Steve is back!!" He sounds much more involved with bass lines and actual playing, and his classic interaction and complementing the guitars (just listen to that TBOS riff)
So even if he stayed and just wrote songs but had someone else play the bass, I think it would sound, I would venture to say, more different than replacing the guitar player or even the vocalist