He sold off his .22 because he WANTED to do so. No explanation needed - his gun/ his choice.
Yes, practice with a .22 will translate SOMEWHAT to other calibres. But only somewhat. Ultimately, for truly important shooting practice, such as for competition or self-defense, practicing with anything other than the gun and cartridge that will be used is not all that useful. Anybody really want to argue that point ? The OP did cite self-defense practice as his (indirect) reasoning for the sale. The issue of the cost of ammo for each is a another matter.
As such, I think it makes perfect sense. Perhaps not to you guys, but it certainly does to me.
I've done much the same thing. That is, sold off a gun (in fact, a couple of them), that I liked and shot well - in order to further a particular purpose. Specifically, so that the two pistols that I would keep would use the same cartridge....and be as close as possible to each other in operation, ergonomics and feel. Given that these two pistols are the ones I carry for SD...and practice with exclusively now.... I think that my reasoning was sound.