Sure but there are huge players in the x86 space so no one company could lock it down to only run companies $X software. Also the supplier and consumers are separate, Intel and AMD make the CPU and other parties like Microsoft, Linux. make the software, and then even other companies sell both the hardware+software (Amazon, Dell, HP, etc.)

If custom ARM chips make x86 irrelevant and are designed 100% in-house then we could have a situation where the only option is running that locked-down/walled OS or using an old, outdated hardware. Sure you can run our own code in the cloud but that's still under the eyes of the cloud provider.

Luckily this probably won't happen. Computation as a product is a large enough market that there should be sufficient demand for an open platform. Even if the market didn't provide this demand, a large number governments would raise concern & intervene.