It's called hedonic adaptation.

> What is actually stopping these model companies

You can say this about any company in the world, selling anything.

It's trivially measurable, and there are people running the same benchmark on the leading models every day and measuring if they degrade. Spoiler: they don't.

But you can always say "the conspiracy goes higher", and that the companies know about these daily benchmarks and are routing them to "quality" envs.