Yes, I’ve seen delays rescinded as well. Mostly at hubs, but also at spokes. Certainly easier for an alternate aircraft assignment at a hub, but also can be possible at outstations, particularly at those not small ones whichever id say LAS is. Not inconceivable that they could swap an aircraft that is coming in from say, DEN or LAX that had a longer layover. Even in the case of delays, you are still supposed to be checked in and ready for boarding on scheduled time.

Also, if your original connection was 79 minutes, and your delay is 45, you still might be able to make your connection. Even if things are as they stand, with the stars aligning relatively well, 34 minutes would be do-able at SFO, IMO, if gates aren’t super far apart, even if not technically within MTC. There’s also the possibility of making up time in the air, etc. that gives you a bit more time in the end. There are things that can extend that arrival time, as well, of course. But it could work out.

You could also see if there are alternates on arriving at LAS. Not sure where you’re going, but is there another connection through, say LAX, that could get you to your destination tonight? Never hurts to ask, and if the delay extends again much more, I would definitely push for that. Or perhaps for routing on A nonstop OAL if available if the delay is MX.