Interesting, any evidence to back this up? The best defensive teams certainly seem to be sticky based on memory (Ravens + steelers dynasties, NE)
There have been multiple studies, but I'm having trouble finding any at the moment. Since 2000, a few teams have been able to string together multiple years of high-end D, but they are exceptions, and most of those teams had elite peaks surrounded by years where they were merely good. Since 2010, Seattle is the only team to have a string Top D's. Others like Denver and Jacksonville fluctuate, and Chicago is unlikely to reproduce their 2018.
When you consider why Offense is more consistent than Defense, the reason is pretty obvious. QB is so much more impactful than any other position, and good QB's remain good until they eventually fall off. There is no analogous position on D, so to remain good, they have to keep more pieces in place, but Defensive results are often skewed by the QB's they play, so a D might look better or worse than it is based on QB's it faces in a given year.
A really bad D can limit the ceiling of a football team, and if it's particularly awful, it can make a good Offense miss the playoffs. The Saints had a run of those types of teams, where the D was just too bad to overcome - even for D. Brees. So, I'm not advocating for ignoring the D or never spending premium assets on that side of the ball. But, if your goal is to build a team that can compete consistently and not just put together the occasional run, you should start with the Offense. Until you have a team that at least has the trajectory of a championship Offense, you should strongly consider why you're diverting premium assets from the Offense. Most would just say that they want a balanced team, but balance is not the same as blanket equality. Your asset spending should be weighted based on what moves the needle and how much. An extreme example would be a team that spends equally on every position. If someone didn't know anything football saw that team, they might say the spending is balanced, but anyone with a passing interest in the sport would find the idea of paying RB's, P's, and K's the same as QB's to be a ridiculous idea (because it is). Likewise, when starting from scratch, as Miami basically is, a balanced approach would consider the overall impact of Offense vs Defense, as well as which positions have the largest impact for each.
The worst approach is ignoring the importance of QB. The next worst is drafting a QB early only to surround him with bad or mediocre help on Offense, while spending premium resources on the other side of the ball.
