PoS is something fundamentally different from PoW. PoS-like ideas were around since before PoW, but such a system can never be used in a trustless way, which was what PoW enabled.

So it's fair to say that Bitcoin could never "transition" to PoS since the whole premise from the start was that it wasn't! It could not exist before. Now that it exist, and has been shown to work, it has caused a cambrian explosion of sorts of protocols with less reliance on trustlessness. But they solve a different problem.