I think practically speaking, as far as fungibility and privacy goes, things like OP_CTV and AnyPrevOut being used for sidechains have effectively the same power as Coinjoin. For most people who aren't trying to launder hundreds of millions of dollars, for which there is no liquidity, there is already a path of fungibility, too. It can get better, of course, but I don't think protocol level coinjoin is a good idea. Just my 2 sats though.