Democrats have already indicated that not one will support Johnson. Last I heard, there are eight Republicans that will not support Johnson, so there will be a fight over who is Speaker. Because the Republicans can never unify over anything, I expect this will last for some period of time.
I do not envision Trump being inaugurated on January 20th. The Dems will sit back and not progress anything over the next couple months.
So, if Chuck Grassley becomes the acting POTUS, at least he is conservative. Hopefully, he will issue a boatload of EOs to end anything Biden established. And a nice side benefit is that he will prove that as a 91 year old, he can accomplish stuff that Biden was clueless about. Biden was senile in 2020 when he was campaigning from his basement.
And if Grassley does become the acting POTUS, I do not see it as a crisis. His age is not a problem, he can utter more than two sentences that are intelligently spoken, which is better than the current fool and his sidekick. The USA will be fine while the idiot Republicans find their posterior using a flashlight.