Under English Common Law, a trial can only continue if there is a meaningful decision to be made. At first blush, this would seem to support your statement, but it goes a little deeper. The meaningful decision does not have to be the primary decision. So if someone is being tried, could be executed with their property being confiscated, and dies. The primary decision of whether to execute him has been made, but the secondary decision about confiscating property is still open.
In this case, the secondary decision over whether trump should be banned from office still has to be made.
More importantly, the Senate makes the decision of how the trial progresses, and even whether the trial progresses. The Constitution is very particular about "SOLE POWER" being vested with the Senate. This means the Supreme Court cannot review the decision.
It all falls to the Republicans to decide. If the Republicans want him tried and convicted, they can do such. If not, they have enough power to block it.