Good ,fun question imo,
In just a few posts thus far I think the answers are covered. Of course we cant ski down without first getting up via the chair lift. Well...........sure rope tows and tee bars and whatever other methods would get us there but its the chair imo that I assume must have lead to the biggest influx of skiing among the general public. Skiing and the whole business of skiing door to door is quite a daunting task as it is for a family and getting up a mountain efficiently would have to had certainly eased that task and lead to many more skiers in genral. But I could be wrong about that. I've used ropes and tees plenty and for those not quite up to the task along with learning the whole ski thing, its the chair imo that would have to make it all so much more appealing vs not, especially for the non avid skiers and beginners.
Snow making also imo is probably every bit as significant as the chair lift for getting people to ski. Especially here in the northeast or other places where snows are not plentiful. Even in places where snow are plentiful I dont have to explain how much help snow making does for extending seasons and making up for mother natures sometimes uncooperative decisions. Just how many winters would so many resorts be out of business and how many less of the general public would even be skiing were it not for snow making. It brought skiing further south than ever and gives those (less fortunate) places a real season instead of just what may often be just a few weeks or couple months at most very many winters and even sometimes barely none.
But boots and bindings also another daunting task of which has eased enormously since plastic boots and step-in bindings. .I do remember lacing up my leather boots and hooking into my bindings. In the end its all we knew so it was normal but once it all became modernized with plastic and gadgetry it certainly was leaps and bounds worth of an advancement. Then ther is the rear entry boots of the 80's and probably into the 90's I think was also (although no longer) another advancement which a lot of skiers really liked for sake of being easy. Many still miss them. I actually wish they still made them and I am not embarrassed to say still used them till not too long ago. I had many years of good and hard performance skiing in rear entries with never an issue at all. Not everyone is a racer and the rear entries were imo more than capable boots for expert recreational skiers too and perhaps a discussion for another thread but I only mention them because I felt they were a significant tech improvement (in line with this thread) and certainly made their mark in skiing even if only for a decade or so. Like them or not they did make things easier so I would say it was a break through to some degree.