I want to know why there would be any political advantage to this
The advantage would be that they would present themselves as being opposed to an attempted coup. Depending on their constituency, that could help them in future elections.
why would they not let Trump leave office with the recent event a real black mark on his record instead of causing new political drama?
There are some that believe that attempting a coup should result in consequences, or, as you put it, "drama". In addition, he can do a lot in two weeks. He can go on a firing binge, issue mass pardons, attempt another coup, etc.
If they decide to cause such a ruckus, it will distract people from
the problem that just happened and the focus will start to turn more
on them and the trouble they are making.
No, carrying on like nothing happen will distract from the seriousness of this. I don't see how you think that taking action against something causes focus on it to decrease, and it's rather disturbing that you're trying to frame this as trouble they are making, when it is Trump that made the trouble. This is some serious blaming the victim propaganda.
Trump is not incapacitated.
Protecting the legislature from insurrection is a duty of the president. Trump failed to discharge the powers of his office to accomplish that. Are you quibbling about the word "unable", reasoning that the 25th Amendment does not apply when the president makes a deliberate choice to not fulfill his duties, rather than not having the ability to do so?
unless it is always possible to remove the president like this,
Yes, the 25th Amendment merely requires that the cabinet declare that the president is unfit. To make the issue of whether he actually is is not an element requires setting some person and/or body as arbiter of the issue, and the cabinet has been so set.
and any vice president can effectively start a coup.
It's not a coup. It is a transfer of power (and temporary one at that) according to the established rules. According to your logic, a parliament voting for someone other than the current Prime Minister is a "coup".