That's fair - young talent can change pretty fast. But the two classes we do know about, the 2018 and 2019 classes, look pretty bad so far.
The issue extends beyond that, too. This team has little second contract talent. You go through the 2015, 2016, 2017 draft classes, those are the guys who should be in their primes or just past. Only two players got extensions from those classes, and one is a long snapper (Mason is the other; Thuney and Wise still could get extensions. Fair to mention that Jonathan Jones, while undrafted, also got an extension, and Adam Butler could as well). The D was very good in 2018 when you had a bunch of key guys (Gilmore, Guy, Hightower) who were 28; those guys are 30 now. Flowers, Van Noy, and Harmon were key players that were gone. Younger guys who were part of that team like Malcom Brown and Danny Shelton didn't live up to what was invested in them, didn't get extensions and are gone. They didn't maintain the D, too many guys left, and they either didn't replace them or the replacements weren't good.
This isn't a "Belichick sucks" rant or anything; I think he's still a great GM and I'm glad he's calling the shots. I think they just hit an inflection point like they had with the D around 2010 when their good guys got old and they had to rebuild; unfortunately, they hit that inflection point on both sides of the ball at the same time. It's not a trivial turnaround.