Based on our current data LK99 isn't a replacement for helium cooled superconductors for the same reason YBCO and similar aren't - critical current/critical magnetic field aren't in the right ranges for what we need out of MRI machines.

I do think it's too early to say one way or the other what all of this ends up looking like, so we might find that purer/larger samples have better properties than what was measured so far, or the discovery puts us on the trail of other RTAPS in the same class that might be better for these purposes.