The only downside is the insane capital liquidity requirement for the ASP.

I'd guess there will be a huge capital fee per round.

Consider this, in Wasabi 2.0 we currently have a remix rate of ~50, meaning that every 1 btc that freshly enters the coinjoin, does on average 50 consecutive rounds.

The ASP has to provide liquidity for every round, not just for a fresh input. So, in our example, the ASP needs 50 btc liquidity for that one user even though he only has 1 btc.