State legislatures acting in the current context would be an extraordinary imposition. This scenario presumably involves the courts, first, rejecting Trump’s legal challenges because they lack the requisite evidence. So the vote counts in the key states would stay the same and there wouldn’t be compelling evidence of massive fraud, and yet the legislatures would act anyway.
The Republicans control the legislatures in all the key states, and they are subject to pressure from Trump and his supporters, but this would be asking them to defy the will of the people as expressed in a vote that would, by this time, have been litigated and perhaps recounted and audited.
One can only guess that the political reaction against this in the states in question would be thermonuclear. This must be one reason why the Republican leader of the state Senate in Pennsylvania, Jake Corman, has so far been steadfast in saying the Legislature is not going down this route.