An interesting conspiracy to believe in is Biden is actually fully lucid but has repented, yet he is still owned by his elite masters who are forcing him to run, so he is intentionally acting senile to give the election to Trump. I don't think it is likely, but it wouldn't be the first time someone has done something like that. A number of saints have.
You can't fake that kind of senility. The part of that conspiracy that might be true though is that Biden is the least motivated candidate ever, the guy is just coasting. But he's not faking his symptoms.