Nikoyay Even if the directive doesn't require that manufacturers lock down their bootloaders, it seems Samsung interpreted it this way (maybe intentionally?).
Maybe? So far I am unaware of statements from Samsung explaining their rationale, though I haven't looked. Have they said they're doing it because of the EU?
Nikoyay What's to keep other OEMs from doing the same?
Right now, probably nothing? All of Apple's devices so far have locked bootloaders, though it is possible to run Asahi Linux on Apple Silicon Macs.
But if nothing is stopping Google from locking bootloaders and nothing is forcing them to, it's not clear why Samsung's choice would cause Google to follow suit. Is Google in the habit of following Samsung policy decisions?
In general I think it's important to distinguish between "Person X says bad thing Y is happening, and Y is really really really bad, so we need to panic even though Person X has presented no evidence of thing Y" from "Person X is presenting plausible evidence that bad thing Y is happening".
No matter how many people echo a scary claim without evidence, if there's no evidence then there's no evidence.