a) The guys in the band are basically in their 60s now. They are way past their prime in every sense of the word, although they are still successful and thriving in their respective areas (Petrucci's line of Music Man guitars are in an all-time high, for example). After almost 40 years of career, they are not just going to find another singer and have him fill up all the expectations of having another man fronting Dream Theater for the short time they have left (they can still keep going for more years, but they're pretty much approaching their endgame as an active, fiercely touring band). It's weird the strong effect James' voice has on the music he performs in. I can very much picture DT, with another singer, sounding like a DT cover band. That's not always a bad thing, but we're speaking of identity and James is a fundamental part of the band's identity.
I agree with this and this has been my stance as well. Very hot take for which I'll probably get tomatoes thrown at: I consider James just as irreplaceable as John Petrucci.
In the literal sense of the word,
anyone can be replaced. Petrucci is not the only one able to play guitar the way he does. Steve Harris is not the only one able to play bass the way he does. But some people are so fundamental to the band they're in that the physical replacement, the mere act of having someone else playing at the same level, doesn't mean anything when you value the weight of a specific fundamental band member in term of songwriting and personality. Petrucci is not the only one in the world that could play guitar in Dream Theater but there's no sense in having Dream Theater without him. And, after an entire carrer (minus the debut album) with James as the voice of DT, and this late in the game as you pointed out, there's no sense in replacing him without drastically altering the entirety of how DT songs sound like.
Sure, there must be dozens of singers out there who could sing the songs and sing them live as well, but none of them would feel right. And as you pointed out, they're not at a level where they can get away with a younger singer 'cause the sheer fame of their classics and nostalgia will carry the shows anyway. For better or worse James is the voice of DT and not even Freddie Mercury reincarnated would feel "right" in his place after having heard those songs sang by him and only him since 1992.