Belknap is not much in the way of precedent and certainly is not a controlling one.
William Belknap was the Secretary of War under President Grant. He was impeached by the House, resigned, and was then tried by the Senate. He argued that the Senate no longer had jurisdiction since he had resigned, and a majority of the Senate voted that they did. He was then acquitted by the Senate and the issue became moot.
I don’t know why you think the majority of the Senate would argue they have no jurisdiction with the current make up of the Senate. They most certainly will use Belknap as an ‘excuse’.