Glenn Hughes, Freddie Mercury (before he passed), and Corey Glover are all freaks of nature who sound (or in the case of Freddie, sounded) better as they got older than they did when they were younger. But they are the exceptions, not the rule.
James is a fantastic singer. Like many who performed operatic vocals for metal bands while in their 20s, doing it in their 50s and 60s after thousands of performances is not realistic. I know a lot of singers who say that as long as you don't abuse your voice, and work it out, it should be fine over the years. I don't agree. Just like anything organic, the older something gets, the more it degrades. And the muscles controlling the voice are no different.
From a live standpoint, James' voice has been hit and miss, even he'd admit that. But fans come into a show with lofty expectations, without remembering that X song was written and recorded 30 years ago. That's a lot of years and miles. It is NOT easy to sing that stuff. The best thing Dream Theater can do to support James, is tune everything high down, and if it just loses its dynamics, drop the song. Even if it is something classic. Maybe rotate one or two songs as an encore, and leave it up to James whether they do those older songs or not based on how he feels during the show.
I was at the I&W show last year in Oakland. James had a rough night, even with stuff being downtuned a half-step. Shit happens. But the effort was there, and you could see it, so it didn't bother me. I am sure James is hard enough on himself without guys like me ranting. We all get old. But as long as the effort to maximize what you have is there (and James makes that effort), it is what it is.
I just hope DT really takes some time prior to the next tour to prioritize downtuning and making sure when they do that, songs still sound right. Tesla fully downtuned for Jeff Keith, and a bunch of their older stuff (Edison's Medicine comes to mind) sounds downright terrible downtuned. But they still play it. That's a big mistake. DT needs to experiment with it, find what works, both for the band and James, and go with it.
But James LaBrie will always be a great singer. He has great technique, and he puts in the effort. Age is a bitch for all of us. He gets a ton of respect from me for going out and giving his all night after night. It has to be humbling when it just isn't there at times. And he soldiers through and gives his best.