LOL the problem with your argument is that you assume everyone is going to premix what they want to send ahead of time.
You can't mix while sending, so yes, you have to premix. The first time you have money on your wallet, you premix them and you can transact with mixed money later.
If someone is so busy that they can't do it, ok.
72 hours to mix 1000 dash through DARKSEND isn't very impressive when there are alternatives that take a second or a few seconds.
It could take minutes or hours, depending the availability of others to (pre)mix, how many laundering cycles are used, etc. It's not a given timeframe. More use will get the times much quicker due to more people mixing. The anonymous coin market share and use is still pretty low at this time.