Er...doesn't anyone play Thieves or Assassins in your game, for whom such disguises can sometimes be really handy?
If I was playing a Thief and one of these came available I'd snap it up in a heartbeat.
Not really, and the same effect could be had by having a vest that flips outward to a different color, or just using a disguise kit to turn a scarf to a belt. You wouldn't spend thousands of gold on it.
Yes, but that doesn't mean it'll be in any more or less overall demand than the Disguiser noted above.
It will be less in demand by non-adventurers.
That was my point. A Staff of Fire is very useful for adventurers, less useful and therefore less valuable to most anyone else. But items that would be in high demand by the populace may not be useful, and therefore not valuable to adventurers. And paying a lot of money for something of trivial usefulness is not something that many people want.
Exactly. You get what you get and then are left to make the best of it. Not everything is going to be exactly what you need and-or can make use of, and if someone else might find it spectacularly valuable, sell it!
Sure, if you can.
But the point is when pricing magical items, when having players go out an buy them, then the price has to account for how valuable it is to players in general.
Not in specific, because if that plate is priced high, that makes perfect sense to the players. It is an item they could see the value of if they had different characters, but it has to be something that they can look at and say "that price makes sense for a PC to buy that effect"
Who the frak else are they going to spend it on, particularly when such spending is almost certain to have been done before the PC ever hears of the rest of the party.
Who says that it has to be before they heard of the rest of the party? Maybe the rest of the party gathered because this guy paid them and bought all their gear. That is a potential origin for a party.
Sorry, but if you-as-DM are letting a PC start with a million g.p. then you've already thrown game balance into the toilet, and have no-one to blame but yourself for whatever ugliness comes next - and it will. Trying to put this on the player(s) to sort out is poor form.
No, it isn't.
I have plenty of players who if I told "Yes, you can hire 50 veterans who will win all your fights for you, turning them all into cutscenes where you don't do anything" They would respond with
"That sounds incredibly boring, I don't want to do that."
Sure, some people want to "win DnD" and so they will do it anyways. But there are plenty of people to whom doing things like that isn't something they are even interested in.
Harder to hit is better than easier to hit, and if you've got 30 mercenaries covering you you're not likely to get within striking range anyway.
Yes, harder to hit is better than easier to hit. And they would have gotten harder to hit by level 3 and then again at level 5 and possibly again by level 9.
With Bounded Accuracy, there isn't really a point where you can't have a certain armor before X Level. It just isn't something that really matters. And Full Plate alone isn't going to push you that far over the edge.
Plus, like I said, the player might decide that they don't WANT 30 mercenaries who protect them from any possible harm. They might find that boring and decide that it would be a stupid thing to do.
All of these just point out hard-fails in the 5e ruleset. There should be rules for hiring mercenaries, retainers, and henches; there should be at least some much better guidelines for turning gold into magic items and-or the reverse, etc.
But there isn't... so why should I judge the game on rules that don't exist?
What else do they get paid in? Silver?
Fame, reputation, land, contacts, favors. I never really needed to give the party who was going to stop Orcus from unleashing a necromantic plague anything. They didn't want to die in the plague, so they went and stopped it. Adding "and you find a convenient pile of 50,000 gold next to the altar" really didn't feel like it mattered.