It reminds me in many ways of Star Trek, which is likewise a patchwork of technological "magic" with equally far-reaching implications.
I do think you're fudging on some level, because of course truly playing everything all out is impossible. I think the point is that if you make common-sense assumptions necessary to play the game, things fall into place. If we were playing a Star Trek rpg, you wouldn't be able to transport into a bank and rob it. In D&D, you can't teleport into a bank and rob it. Why? Either wealth isn't stored that way, or it's protected somehow. What rationale you come up with is less important than the simple fact that when a player tries to use the supernatural travel method of choice to lay waste to the world's economy, that isn't going to work.
If you haven't thought about it, it might not be obvious, and a DM might read the Teleport spell, see nothing in the spell description itself to invalidate this type of plan, and let it go forward. Thus, advice.
That's a good analogy.
However, I wouldn't say you necessarily couldn't do that in all D&D settings - how common Teleport is, how real a threat it is relative to other threats, and how worthwhile protecting against, it is, matters. If a handful of Wizards in the world can cast it, banks probably rate protecting against it similarly to protecting against tunnelling or the like IRL - which is to say it's low on the priority list - but if the bank is big enough, they will have it.
Further, banks in medieval settings are typically protected more by reputation than defences - after all, no bank can stop a few dozen soldiers getting together and robbing it - but if it's owned by the crown and it's known that vengeance will follow, they will likely be discouraged.
That said, if it's just a straightforward and reasonably-priced spell or ritual to ward an area against teleportation or similar magic, then that'll be done.
I totally disagree with your suggestion that the rationale doesn't matter though - on the contrary, the rationale is more important than preventing a PC getting a lot of cash/ Which, let's be real, he won't even be easily able to spend - you need to think about the consequences of actions, not just preventing actions with DM fiat - all that stolen money will have to be spent very carefully if he doesn't want literally hundred of angry assassins, curses, dragons and so on descending upon him - so carefully, in fact, that's probably his life's work.
So I would suggest two things:
1) Just using DM fiat to block things is very short-sighted DMing. When the player or PCs comes up with a terrible plan like this, just spend some time thinking through the consequences, and, if you want, hint at those consequences (as in reality, the PC would have hours or days to think about it, not the minutes they likely do at the table), and if they go through with it, inflict those consequences. And smile.
2) If you do need to use DM fiat (I know you hate this term, but that's what it is, and sometimes it's right to use it!) to block something on the fly, stop and think through the consequences for the setting. You don't want to have a setting that is a godawful mess like Star Trek, where nonsense-particles are used by terrible, shamefully bad script-writers to arbitrarily inflict scenarios which make no sense on the characters. Do not emulate that, I say. That's like seeing the original 90210 as the level of story you want to emulate or something. I love Star Trek, but sometimes TNG and Voyager (and ENT and even rarely DS9) needed a good spanking for needless use of nonsense-particles, just because the writers were too lazy/dim-witted to come up with a story that actually USED the technology in that world.
Anyway, I'm getting off topic - if you do use fiat to block something, think about what you are doing. For example, if you say the vault has a magical ward against teleportation, that's fine, but then you need to think about how the ward was created, how easy it is to create others, who did it, and so on - these might not be questions for during the game (but they might), but they are questions for you and your setting, post-game.
I speak from experience here, specific experience, too, as I run a 4E game which has a lot of heists, and I use a mixture of making-stuff-up and consequences to direct the PCs. I don't suddenly introduce fiat elements unless it would be truly stupid not to (which is once in a blue moon). Instead, I'm more likely to say, allow this bank to be robbed, but ensure future/other bank designs defeat that (not in a fiat "on the spot", way, but in a prepared "in my notes" way).
Still, you may need to use fiat to block something like that, and that's okay - but DO consider the consequences for the world - your advice that the rationale doesn't matter is bad.