Maybe for small amounts if there's a useful application for it, just like I'm okay having small amounts of Lightning funds in a custodial wallet.
For a relatively long time?
I've had funds in a custodial LN wallet
for over 2 years. I actually lost a small amount due to an uninstalled wallet for which I can't find back the picture I took of the backup QR-code, which could be considered profit for the wallet, but they can never know if the user won't find back the backup years later.
So really there would be little to no difference between us and any other custodial, centralized mixer. If you think about it this is also exactly what you are doing with more "traditional" mixers: you're swapping real** bitcoins for a promise of future** real bitcoins.
But the "traditional" promise doesn't expire: you get your Bitcoins after a certain amount of time.
blindmixer is really no different just because we have packaged it in a more sophisticated way. you swap real bitcoins for giftcards (a promise to future real bitcoins), and you can immediately at will converse those back to real bitcoins.
Unless you forget about them for too long. How about an alternative: instead of expiring instantly at a certain moment, could you charge a daily fee for storing the value of the giftcard? If you charge for example 0.02% of the value per day, that amounts to 7% per year and in exchange giftcards never have to expire.