Regarding weather at bases being a factor: I have to wonder what effect the SOP (and changes/additions to it) might have on that too. My recollection from flight school is there was a lot of weather that was technically (and legally) flyable, but it wasn't possible to fly because of the way the rules were written. A lot of these bases were churning out students at a crazy clip 80 years ago, and 50 years ago, and 30 years ago... have to wonder, has weather really changed that much?
We crash a lot less than we used to, which is a good thing.
Nevertheless, there might be areas where aversion to risk is a factor in training delays. Not saying "launch the solos" (nobody wants to go back to losing 500 airplanes a year), but maybe there is some amount of trade-space between where we are and operating more efficiently, without sacrificing flight safety.
Flying in a variety of weather conditions can be a good confidence-builder for students, too. It just has to be done in the right way, so they don't get cavalier about it.
The other factor I wonder about is length of syllabus, as compared to prior generations.