Why would Ryan do this?
If Clinton was going to be indicted, then he might have a free run, but otherwise he's a known quantity with plenty of neo-liberal free market bs baggage (e.g., various crap budgets, 2012/Romney) and would be very unlikely to beat Hillary.
Right now, he's in a position of incredible power in the US governmental system and if instead of fighting Trump, they united with him in some way, the repubs are in with a good shot of keeping the Senate in repub hands (the house is a gerrymandered lock). If they lose this, he doesn't take a lot of personal damage (blame Trump), keeps his current position and will be positioned for a much stronger presidential challenge in the future. I think Ryan is a bit of a scumbag but I recognize him as a smart man also and can't imagine him taking such a risky chance.