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....
http://coinmarketcap.com/currencies/monero/ (click on "all" for the full history)
You can implement the exact same distributed and trustless mixing mechanisms in wallets that live on top of Bitcoin
No you can't. It is totally different cryptography at the transaction level, and would require at least a soft fork, but that is very messy and far more complex than any existing or propose soft forks, meaning it has essentially zero chance to happen in practice. It could and probably will happen on a side chain, but that raises various other issues.
There are other differences like an actual deployed (and somewhat stress tested) mechanism for managing the blocksize without a hard limit.
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 don't need distributed trustless mixing mechanisms in the protocol to have that, you can implement the same in wallets. I remain unimpressed by these bitcoin clones that are simply adding simple functionality to the core protocol, functionality that can easily be layered on top of bitcoin. That is not an improvement IMHO.
Zerocoin is an example of a true innovation as I see it. Zerocoin does not rely on mixing as Monero does, but implements true removal of all history. This is something that neither Bitcoin nor Monero can do. If someone made this, I'd seriously consider it.