I'm not so sure about this. For starters, millennials (Gen Y) are currently 26-40. These folks are actively buying timeshares right now.
A Consumer Reports article from 2016 quotes:
Those folks are almost certainly web-search-literate; the use of the word "google" as a verb appeared in popular culture
as early as 2002. Likewise, there are already plenty of timeshares that have little or no resale value, have similarly cosmetic differences between resale and retail, yet still sell ownerships at full-freight. Wyndham is a good example, but there are others. So, the information is there, and the differences are stark and usually very clear: a secondary-market purchase is just a better deal. So, why do people still buy from developers? My guess: people who buy timeshares
do not want to know that they spent more than they could have. If someone doesn't want to know something, it doesn't matter what the facts are. That sounds hard to believe, but bear with me.
Think about how the business works. Timeshares are a product that is sold, not bought. Very few people go on vacation with a plan to buy a timeshare---DISboards company excepted, of course.

. Instead, a family is on vacation having the time of their lives. They take a tour, where they are told that they can bottle this magical feeling they have forever, "vacationing for decades to come at the cost of a handful of trips like today's." This sounds like a grand idea---and, really, it is! But it is also just a little bit aspirational: it means taking more vacations, more often, in a little more luxury than they are used to. And this possibility is presented to them in a way that has payments that seem affordable for the next few years. So they buy.
Now, you have a new buyer who feels
really good about the decision they just made. Maybe they have kids who are in grade school, and they are imagining taking them on vacation every year in a way that they never have before. Maybe their kids are older and they are thinking about grandchildren down the pike. Maybe they are about to become empty nesters and imaging all the travel they've put off while they've been raising a family. Suddenly there is an easy way to do whatever it is that they are thinking about.
That person is probably not going to spend the next several days trying to come up with reasons why this good decision they just made was a bad idea. And, once that period passes, the timeshare is irrevocably theirs. Of course,
some will, because it is a lot of money. Some of those will see the cosmetic differences between resale and retail and convince themselves that they were right all along. Some will rescind. But, few enough will rescind that the overall business model still works for the developer, and the world keeps spinning.