It's bad to create a dangerous performance cliff. There could be some TCO based code that works fine, and then a junior coder makes an 'innocent' change that makes it ineligible for TCO, then that code eventually starts getting OOM crashes on heavy data.

I think if they're gonna do TCO then it really should be syntactic. If someone is writing their code around an assumption of TCO, then they almost always want an explicit guarantee of TCO, and they want to fail fast if TCO isn't happening.