> if the Republicans control all three branches of government, that they would not create a federal law that bans abortions everywhere?

If the voting public of the US shifts to where it adopts a view you disagree with then of course the democratic institution would continue to reflect the public.

But we have good reason to believe this will never happen. Polling shows that most individuals support a woman's right to abortion. And even if the Republican party gained control of all 3 branches, they aren't a monolithic and unified block when it comes to policy. "States rights" is a HUGE motivation for most Republicans, and a federal policy banning abortion would be a big stumbling block for getting the votes to pass such legislation even with a Republican majority.