If you think Valve will exit software development, then you don't understand the market. Valve did do that and had this luxury when they were the only big game platform on PC. Unfortunately for them, there's GOG, EPIC, and MS Game Pass now. They need exclusives and the best way to do that without going the EPIC route is by developing their own games.
I'd argue that Valve doesn't need exclusives just yet. By virtue of being the default PC store they get plenty of games that are not on any other store. But this will change.
