Yeah, would be nice, right? But then there's these ... people you have to pay that have their bitcoins on exchanges that don't allow rotating addresses, so all your privacy goes down the drain as you pay to the same address every now and then. There's only so much you can ask from people you owe money ...
Discussion
what if you pre-mixed/split into very many UTXOs, one per separate address, that you keep around and never send from the same one to one of these single-address recipients?
I realize that's a shitload of work and fees.