Realistically, the chances of parlaying together a bunch of elite WR seasons while constantly trading back for the younger guy is virtually impossible to sustain. At BEST, you're looking at going through an on again off again approach where every couple years you have a young stud WR followed by a young up and comer on your bench who is eventually going to become a stud but isn't really helping you out in the meantime, assuming that your young but aging stud enters his trade window off the back of an elite season. Meanwhile, you have the potential for the whole thing to get blown up completely at any time if any of these 50/50 shots end up being a bust.
I think you're missing a few key details.
Since established stars in their prime usually have more trade value than all but the very most hyped rookies, you're not making a straight up exchange. When I see teams making these deals, usually the team getting the younger player also gets a pick or two thrown in. Look at this thread. Nobody is trading LeSean McCoy for Gio Bernard, but you might see someone trade LeSean McCoy for Gio Bernard + draft picks. Those draft picks are an important part of this type of deal. They help offset the risk and increase the profit margin.
As far as the roster holes created by taking on developmental players, there's some truth in that. However, it's not really a valid criticism in the long term. Let's say you have the option of keeping Brandon Marshall for the next 4 years or trading him for DeAndre Hopkins and a 2nd round rookie pick. If you take the Marshall side, you get 4 years of useful production and then a giant hole in your roster. If you take the Hopkins side, you get 4 years of less useful production and 4 years from now when he's 25 he will command a higher trade value than Marshall. Then you can trade him for the 2017 version of Hopkins and repeat the cycle.
If you stick with this strategy and make the right assessments, the value of your roster will never go down. The people who ride Marshall until he retires will have to use their assets to acquire a replacement just to maintain their roster value whereas you can use all of those same assets (rookie picks, trade bait, etc) to ADD to your roster value. Stick with that strategy over 5-6 years and you should eventually see separation from the pack. The fact that you're constantly accumulating means that over time, you have so much value on your roster that you can start a contender while simultaneously grooming the next generation.
I think a lot of the critics and people who argue against this don't fully appreciate what I'm saying, which is not to trade every RB who turns 26 or every WR who turns 27-28. Players like Marshall and VJax who fill up the box score every week are like gold. If you look at any cluster of "next big thing candidates," very few of those guys are ever going to reach that level. That's why you see so many of the youth-crazy dynasty owners crash and burn while the teams who are willing to collect expiring stars tend to make the playoffs. I only recommend making the youth-crazy moves in cases where your degree of certainty is large enough to cover the margin for error. I.e. I would not trade Jason Witten for Gavin Escobar, but I would trade him for Tyler Eifert. Eifert has more long term value potential and I have enough confidence in him realizing that potential to a great enough extent that he offers more value moving forward than Witten. Gavin Escobar also has more long term value potential than Jason Witten, but since I think it's fairly unlikely that he ever realizes that potential, I'd take the instant points and keep Witten.
You'll note that when I use personal examples of trying to trade Fitzgerald and Marshall I was targeting absolute top tier prospects like Dez and Blackmon. I would only give away a proven star if I was 90% sure I was getting one back. I think you need to be extremely careful with these exchanges, lest you end up with a roster full of Braylon Edwards, Sidney Rice, and Roy Williams. Nobody is saying that this is easy. All I'm saying is that in instances where you can make the correct assessment, it is absolutely profitable to make these trades. We can argue about how good people really are at making these assessments, but the underlying math in an ideal hypothetical is beyond refutation. So from that standpoint any time you can make a 100% accurate read on a prospect and act accordingly by moving an equivalent older player, it is hugely profitable. Whether or not anyone can really do that is a lot more dubious.
) took Mark Ingram ahead of Julio Jones and AJ Green in one league before their rookie season and promptly traded him the next summer because I thought he looked like crap? No. To say that I'm stubborn and rigid because I still think Jonathan Stewart might have some good years ahead of him is kind of silly when I've jumped ship on guys like Jon Baldwin and Mark Ingram more recently despite being pretty high on them at one point.
