I agree that we should concentrate on the essential form.
If the essential form has problems, the rest becomes almost irrelevant.
If we try to envision the consequences of these problems, we could have a new kind of Sedevacantism. I mean, a layman can be elected Pope, but to be actually a Pope he needs to be a Bishop, right? I imagine that some traditional theologian has adressed this issue.
So, if Benedict XVI was only a Priest (his espiscopacy comes from the new rite), and Francis is a layman (both his priesthood and episcopacy comes from the new rites), then, John Paul II was the last Pope.
This is probably why the SSPX published that super long article back in 2006, I believe, saying that the new rites are valid. It was written by a Dominican from Avrille.