It's not going to ruin the game if we see an insight into Adult Master Xehanort's life.
If it takes away from other elements that need more insight, screentime, and development, it could.
Aside from just filling out Xehanort's life, what would this really bring that is
crucial to KHIII? You mention how Adult Xehanort could have learned stuff in his travels, but, naturally, that would be information his older self already has. Anything that happens to Adult Xehanort is stuff that MX already knows about, already went through, etc.
The key characters of Kingdom Hearts X were never set up in any prior games when they were first revealed yet they fitted into the lore. I've been stressing to let Kingdom Hearts III be that development platform for Young Adult Xehanort, but at this point, it's clearly evidence that your distaste for this idea clouds you from the alternate elements that could be introduced in Xehanort's character development.
And imagine if all the X characters were only introduced in KHIII, where they're important to the plot. KHx is the set up for KHIII. Like what about this don't you get? We're talking about KHIII here, and all of these other things are set-up for the plot in it. Except you're arguing that Adult Xehanort should be introduced, made important, etc. all within KHIII with no set-up. That's the problem, and that's why it's unlikely. And nothing clouds my judgement. I've been fair and balanced this whole time and never even said it was impossible. All I gave was reasons why it was unlikely, and potentially unnecessary given what we know.