Most gacha games will eventually release some sort of ultra rare universal upgrade material (like a universal constellation juice), but they have to be really careful with them. If whales decide that their whaling has been undermined by freebie handouts, they will stop spending, which publishers definitely do not want. I could see something like that being released yearly on anniversaries or major version updates (like 2.0, 3.0 etc).
That said, for 4* characters, you will have the monthly rotating shop characters to guarantee yourself a constellation sooner or later, as none of the 4*s have been limited so far. I don't necessarily mind dropping some primos on a rate up banner to get some 4*s though, in my experience in the Childe banner (the only one I pulled a lot in), it was fairly easy to get copies. Unless you're just rolling all of your gems as you get them instead of saving them for a character you want, I think you'll be fine getting at least C2 of new 4*s - if that's your goal.