If Trump had held off with Biden, he would still be the expected nominee at the convention.
The debate is what sealed Biden’s fate. He did it to himself. Trump did not cause the debacle nor did he cause the withdrawal of Biden. That was in the cards as played by the puppet masters.
My theory is that the Democrats insisted on holding the first debate so early in the year because they knew Biden's cognitive abilities were beginning to deteriorate at an even more rapid pace. Their hope was that Biden had enough left to turn in at least a mediocre debate without any major flubs. The Dems and their flunkies in the media would then praise Biden for a "string performance", claim he was the clear winner, and no further debates would be necessary. They could then keep him under wraps until election day. Instead, it blew up in their faces.
+1