The answer to this question is clear and spoken by the Speaker. She says 'this is not about President Trump, the voters will decide that', and she is correct. She knows she cannot get something like that through this Congress and the President certainly won't sign it. His Cabinet and current VP would not agree to executive the Constitutional process either against the President. So the plan is for 2021. If Trump wins, and the Democrats capture the Senate, they can possibly override a Presidential veto and then use this to get rid of Trump. Fact is they wouldn't need the 25th amendment, or the bill they propose to do that. With both Houses of Congress in Democrat hands, they can simply impeach him and remove him from office this time. But, I guess from their standpoint, it is good to have another process, since they have proven they will not stop trying to remove President Trump anyway possible. The other 2021 usefulness of this bill is what has long been expected of the Democrat plans for the Presidential ticket of Biden/Harris, to remove Biden shortly after inauguration, allowing V.P. Harris to become President. This process would certainly help with that, assuming Biden is not in agreement with the plan or has changed his mind. Donald Trump obviously agrees with that idea:

Crazy Nancy Pelosi is looking at the 25th Amendment in order to replace Joe Biden with Kamala Harris. The Dems want that to happen fast because Sleepy Joe is out of it!!!

— Donald J. Trump (@realDonaldTrump) October 9, 2020

Below is an article from RollCall which goes into the question of "Why is Pelosi suddenly talking about the 25th Amendment?" Speaker backs Raskin bill to set up nonpartisan commission to determine a president’s fitness, says it’s not about Trump Raskin’s legislation, an updated version of a bill he first introduced in 2017, would establish such a body: a 17-member commission composed of four physicians, four psychiatrists and eight former high-ranking executive branch officers appointed by congressional leadership from both parties. The final member would be selected by the other 16 to serve as chair of the commission. More From RollCall: