Monero has lost ~90% of it's value vs. BTC since last September
Uh, no. You are misreading something.
My bad, I was looking at the market cap valuation which is off that much. The BTC price is only down ~75% from last Sept. Still not exactly great price performance....
You can't really blame XMR (which is priced against BTC, and more fundamentally trades relative to the greater crypto ecosystem) for BTC being down 75%. XMR is down relative to BTC over that time period (but up >100% since December, so why cherry pick starting points here?) but not anywhere near 90%.
I disagree here, there are multiple proposals on how to layer privacy oriented transactions on top of bitcoin. Some rely on shared secrets, some enable wallets to communicate and build distributed trustless mixing mechanisms (which is exactly what Monero says it does).
You are incorrect. That isn't exactly what Monero does. Monero doesn't do "distributed" mixing, it does counterparty-less mixing. Which means nothing distributed between any parties (or using any kind coordinating "node") and no shared secrets. There is no other proposed solution to do that. It can't be done in wallets.