But not the actual legislatures themselves. A random state senator doesn't have the authority to appoint electors. Or to send them to the electoral college. Nor do they have any legal authority to 'decertify' any election.
The power to assign electors is granted to the state legislatures. And not ONE State legislature has found that their own elections were illegitimate of fraudulent.
Nor has any court found this. Nor has any count, recount, hand count, forensic audit or electoral vote.
So the election was illegitmate according to who? You've cited no one with any legal authority.