Using an altcoin as a bitcoin mixer largely suffers from the same issues as any other bitcoin mixer, and I agree in practice it may be worse. You are still going through some kind of chokepoints (exchanges, etc.) which will keep logs, and as you say volume will be an issue (though not so much if you are willing to do it slowly over time; same with bitcoin mixers).
Good anonymous coins simply eliminate the need for mixers altogether by keeping the blockchain opaque. If I wanted to move 7000 BTC worth of XMR privately as XMR (not converting it) I literally could do it quite easily even though that's something like half of the total capitalization of the coin.
Obviously 7000 BTC worth of XMR is unlikely unless XMR gets a lot bigger.
You can move, but you can't sell them and thats the issue. Recall the NXT hack from Bter, the hacker was willing for a cut price deal rather than sell them.
I agreed with you that moving funds through (meaning into then out of) an altcoin as a means of using that altcoin as a mixer is not a good use case.
An anonymous coin by itself is not attractive enough to become big enough. If it is something else which becomes big and happens to have the anonymity feature then that can be used later.
This is where I'm not sure. Forget about the idea of using the alt as a mixer and consider that people may simply value their privacy and not want to go through the trouble of a special "mixing" process at all. Sure if you are a hacker and you want to wash 7000 stolen BTC then going through mixers, gambling sites, exchanges, etc. are fine.
Normal people, dealing in smaller amounts where the liquidity of many alts is perfectly adequate, or even routine modest-sized business transactions, aren't going to do that and may still not want their affairs broadcast out on a public blockchain. They may prefer to simply use a monetary system that doesn't have so much built-in visibility to begin with that you need mixers.
Even if this is only a minority of people, that may still be a very large market, just as there is a large market for financial privacy in the conventional banking system.