Ah, one of my favorite subject matters to discuss (and one of the most vicious, as well).
The thin line that separates the objective from the subjective, as elusive as ever. I think there's a couple things we can all mostly agree upon:
1) James is probably one of the most recognizable voices in all of metal and progressive rock. It's not a matter of technique or even range (one of the most overvalued characteristics that people often talk about in metal music), it's a matter of timbre.
2) James has struggled quite a lot in his career as a singer in many ways. Being DT's singer is probably one of the hardest jobs to cover in the metal and progressive realms. Even though there are bands, nowadays, that surpass DT in sheer technicality (assuming we all agree upon what "technical" means, which in metal is usually the music that's hardest to perform), DT is still pretty much the "band of virtuosos" to the mainstream. The band with awkwardly long songs and with a singer that occasionally sings in them.
On the other hand, you have the food poisoning incident, the incredibly taxing touring schedules and the aging process. Time has not been very kind to James, although he has clearly and openly made all in his ability to keep up (I've listened to hours of podcasts of him basically talking about vocal technique and care. The man clearly knows his stuff, whether you like it or not).
3) His live performances, of late, have been mostly subpar. This is coming from a vivid defensor of James (I've sustained long and incredibly absurd discussions with people on the internet throughout the years), but clearly his stamina is not what DT's regular touring schedules asks of him (some of his performances I've listened from the last Dreamsonic tour were not good at all, and it pains me to say so), but that doesn't mean that DT should change singers (I'll elaborate on this in a bit), and clearly Petrucci knows this (the undisputed leader of today's DT, even though Portnoy is back on the fold).
Out with the objective, in with the subjective...
Even though James' faults of the recent years, DT would not benefit from letting him go. All these folks that keep hammering him and insisting that DT should fire him and get a proper replacement, clearly know more about the internal, personal and profesional affairs of a band than the people actually running the band.
Personal relationships are so fundamental to the inner workings of a band, that even though DT had a pretty successful run with Mike Mangini, they eventually gravitated towards their safe space once again: Mike Portnoy. Not because Portnoy's the better musician or drummer (which is something entirely subjective), but because the band's relationship with him is just in another level which will probably span out their most commercially successful record in years (not that their previous efforts haven't been successful, but DT's not a band that relies on commercial success to exist because they've built a fan base that's not going anywhere).
James has been the voice of Dream Theater for over 30 years, and basically has helped sculpt the sonic stamp that definitely makes the band still stand out over the dozens of DT clone bands and generic Progressive Metal nonsense with singers that timbrically sound so similar. I'm aware people will disagree with me on this (makes me think of the guy that runs the DT Archive YouTube channel that takes shots towards James every time he has the chance), but I stand my ground on this. A couple of things to take into consideration:
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.
b) I know that other bands have replaced their singers with younger counterparts (Journey comes to mind), but DT is not as big of a band as to magically work with another, younger member. Clearly everyone there (including Mike Portnoy) has profound relationships and connections with James, and if they feel comfortable with him still fronting the band (in both his good and bad days), that's pretty much what we're going to get.
I agree that James' tuning has become inconsistent, to say the least, but that doesn't mean he has lost his pitch. There's a lot of informal recordings of him singing a'cappella and pretty much nailing everything with very precise tuning. It's his technique I'm most worried about, because I feel that he has given up tone in exchange of lasting for 2 or 3 hours of live shows. James' high range is very compromised these days, and that's the reason his tone is not good when he approaches that zone, because he'd rather hit the note but not think about articulation and tone of the note (that's why his upper range sounds so nasal, because of the abuse of his nasal cavities to amplify his sound). His lower and mid ranges, however, are pretty much still phenomenal. I even recall one of his recent "Pull Me Under" performances in which he just said "fuck it" and sang the whole "watch the sparrow falling..." verse an octave lower. Sounds weird because I've been listening to that part on the high register for 20 years, but if it makes him feel good and helps him produce a better tone, I'm all for it. I've seen and heard that song live dozens of times, so I don't care if James doesn't get up there if that helps him get a better hold of his performance.
All in all, I don't see the band replacing him any time soon, but I feel it's healthy to focus on his strengths rather than his weaknesses. The band clearly takes that route every time they tour, and somehow I feel it's for the better.