Oh no, for you as the player, you can own as many as you want - but you can only PLAY one at a time. At the same time, publishers can't have infinite games lieing around, because they must sustain them with server-infrastructure and tech support, as well as maintain them with balancing and patching.
These are undebatable truths, absolute facts if you will. Now from those we get interesting developments when we enter the world of Massive-Multiplayer-Online games. MMOs require a critical mass of players to not only be functional, but also have some "fluidity". In leage terms that means having so many players, people never need to wait too long, no matter on what skill level they queue for a game. On a more extended level this also affects the fan community and their creative output in art and writing, and general meme-shitposting, as all that is free advertisment. That last part is ESPECIALLY relevant in the Free-2-Play segment, which requires a high spikes on players to be profitable.
And it all boils down to profit, because a game that generates no profit cannot be supported, and will cease to exist. Doesn't matter if you have an account or how high your rank is, or how good the game is, if the servers shut down because the studio gets no money, it's game over. And at worst the Studio can't pay it's employees and it disbands.
So obviously when you have a NEW F2P title coming, it requires a massive growth over a short time at release, or it will be a tremendous financial failure for the studio. In the current industry you either shine like a diamond at release, or be forgotten under mountains of new games that also release every year. Comeback-stories from that oblivion are so rare you might as well treat them as absurd anomalies.
Also, taking over already existing infrastructure for the new project lessens the financial burden and the risk connected to it. So by eliminating their biggest competition, which ironically is their own already existing game, as it undoubtitly WAS the biggest thing in the western hemisphere for some years, they hit two birds with one stone: make sure their main taregte audience is not tied to another game, while at the same time saving a lot of money by re-using their infrastructure.
It's a simple consequence of basic facts in now the Gaming Industry works. But hey, you ran out of arguments LONG ago given how you answer by intentionally misunderstanding things - tbh I could tell you the sky is blue and you'd demand literature at this point. Stay in denial a bit longer, at least Riot will be happy to know that enough morons go along with their plans and reward them for it.