Here's an econ paper that studies that question: https://www.researchgate.net/publication/342185292_Is_Bitcoi...
FWIW, I don't have a strong opinion on the evidence presented in the paper -- the analyses seem sensible, but this isn't my field of expertise, so I'd be hard pressed to point out, for example, what alternative analyses they could / should have done.
Also, it's not even obvious to me that unbacked Tether causing the BTC price rallies is necessarily a reason to pull out; markets are weird.