Wow as a Kamara owner I may have to go shopping for Saquon after he declares.
Before getting into specifics, more generally while truly elite RB prospects aren't infallible, they have a very high hit rate at least in the short term. If a rookie RB is a good enough prospect to have an ADP in the first 3 rounds of redraft leagues, their dynasty value appreciates from that year to the next 86% of the time. That is a huge percentage in fantasy football and a better return than you can get on basically any subset of players, even if it is a relatively small sample size.
Likewise, as I've touted many times 2nd year rookies coming off a good rookie year (especially ones that weren't elite prospects) are among the most tenuous and dangerous assets in all of fantasy football. The list of busts that had "proven it on the NFL field" for one year is unconcionably large. One year is "proof" of nothing in the NFL, but players that have done it for one year have the value as if they had done it for 7 straight years but were still only 23 years old.
More specifically about Kamara, he did look great out there but this was just about the best situation imaginable efficiency wise. He already landed in the perfect spot for a non-lead RB, a team that had made RB1's out of far less talented players before. And that perfect spot is tenuous going forward, with Brees aging and Payton constantly moving on and off the hot seat.
Not only that, even among that perfect situation for a guy like Kamara (which may or may not last) this was a particularly good year for it. Mark Ingram set career highs in rushing yards, TDs, receptions, and receiving yards. He didn't just suddenly and coincidentally get better at all of those things in his 7th year in the league. Everything just lined up this year.
I like Kamara a lot, but I'm with those that say they wouldn't be surprised if this ended up being the best season of his career, and I think we've almost certainly seen his career high in TDs already. There is a reason there aren't many part time backs that have ever held the long term value of guys like Zeke, Bell, Peterson, Lynch, etc. It's just not sustaininable. He scored TDs at a rate significantly better than Ladainian Tomlinson and his double digit YPR this year makes Marshall Faulk look like a fullback and would be the best in NFL history if he maintained it for his career (hint: He won't, many guys have done it for a year or two but none for a whole career).
In reality, he's not some crazy combination of Ladainian Tomlinson and Marshall Faulk. So the only way this guy ever sits in that consistent top tier of RBs long term is if he becomes a feature back. That is certainly a possibility, but there is a ton of risk in passing up on a guy like Barkley in hopes that he becomes that. And unlike the popular assumption that even if he doesn't become that he will still be a strong RB1 long-term, that's just not a reality because this efficiency is not sustainable.