EDIT: merel saying its a problem is not providing a reason (but it this way I UNDERSTAND the potential issue but have yet to get given any good reason to believe its a big issue; the outside ombat utility thing)
People have explained in detail, dude.
In 5E, one of the big limitations on the power of casters is that they can only Prepare or Memorize or have ready a certain number of spells. This is a major limiting factor. They have to guess what spells they will need for that day (or, with Sorcerers, for the rest of their career, barring a few change-ups).
Very often you will guess wrong, meaning they are limited in power/flexibility.
This is a very much bigger limitation than the number of spell slots they have. Particularly at higher levels. Your spell completely negates that limitation. Even at two levels down, like I was saying, it would be a really good spell, because even being able to cast any cantrip you want is pretty great, because there are situations where any given cantrip just solves the problem.
At one level down? It's insane.
Even ignoring combat, basically every situation where a spell might have been useful, you can just cast that spell, and at a low cost. Literally every problem the PCs encounter will get solved by magic. If that's what you want, if you want skills to not matter, differences between what spells PCs prepare to not matter, and every problem to be solved by just casting going through the PHB and casting Anyspell, great, but that's what'll happen.
yeah.. thats what rituals have done anyways...
Dude. Count how many spells are rituals. See what spells are rituals. Notice how they're lower-powered spells which have very specific applications and there are very few of them.
If you had "Anyritual" as a spell, which let you cast any ritual-castable lower-level spell, even ones you didn't have prepared, as as a spell, then I don't think people would be telling you how broken it was. But you want to be able to cast literally any spell on their spell list (including ones they don't know), and there are loads of spells which aren't rituals
for a reason.