Imagine wasting the biggest historical enemy of the Justice League, the "original" Thanos, in the fifth movie.

It's neither a waste nor a dumb idea because WB wasn't simply recreating the MCU. This was supposed to be a series of 4 megablockbusters (think Pirates 2 & 3) each making over a billion dollars with anticipation building on each film. 5 films (or 5-7 given spinoffs) isn't an insignificant number of films for a "core franchise." Even the gold standard of franchises was only able to squeeze 8 or 9 films out of RDJ.

WB & Snyder's vision is clearly better understood as a universe built around a major series with spinoffs radiating out from the center. In the wake of Nolan's Batman, DC didn't need to do anything to build interest in the core DC brand. They were in a position where they could have started out on top.

They HAD to stop Snyder

Yeah, but they had to stop him for the exact same reason why a successful version of what Snyder wanted to do would have at least topped Infinity War. Snyder wanted to push all of the chips in on his specific vision.