The DASH mixing has a fee, isn't that useful Sybil protection? An attacker can keep doing that only for so long before his resources are exhausted.
Yes but since there is full simultaneity the attacker only needs to do that during the time period of interest. It may be that for whatever reasons of my own, I'm interesting in attacking today's transactions but not tomorrows or next years (necessarily). That has a finite cost (and probably not very high in practice).
The DASH mixing is "pre-mixing", i.e. you can mix your coins today, and spend them privately next month. So when there is this period of interest, the transactions happening could be coins mixed a year ago.
Yes it could, or it could not. It depends. In particular you can't possibly have mixed the coins last year if you just received them. So that puts a bound on it if a particular "target" is of interest.
Heuristic wallet could detect when there are 2x more mixing happening in the network than usual
That sounds pretty unworkable given normal fluctuations in honest usage (or at least not user friendly and introducing even more delays into mixing that I'm told is already pretty slow), and it also introduces an easy denial-of-service attack, but who knows maybe something like that could be made to work. It is hard to say without a specific proposal and probably field testing.
Or you could just use Bitcoin which has vastly more usage to begin with.
.... Monero is the most trusted, and most used anonymous cryptocurrency on the planet. 