Because a lot of what made our defense as good as it was over a stretch of the season isn't something that is often sustainable from season to season. Phil Steele uses the metric of turnovers across NFL and CFB where turnovers are a result of what he calls "good fortune"...teams that have a turnover margin of 11+ in one season, the next, are often "weaker" the next. On the flipside, teams that have -11 turnovers in one season, the next, are often "stronger" the next. Are there outliers? Absolutely. But relying on turnovers isn't a sustainable, predictable metric, and it drove a lot of the success we had last year.
That isn't to say our team will be weaker, but the defense surely will be. Miami can offset a lot of those trends because they have some positive trends like a veteran QB where cutting down on his turnovers and raising his completion percentage will go a long way to this...but also a weaker schedule with a lot of favorable games for the team.
http://plus.philsteele.com/Blogs/Blog_PDFs_Images/2018/DBJuly24/TO_Equal_Turnaround.pdf
cfbstats.com - 2017 National Team Leaders
Also, I am still very concerned with how the team ultimately plays the run and its inability to get off the field on third down. Couple it with poor red zone defense and there are just some issues - possibly schematically - that I think keep us from being elite. Hopefully there is some improvement (some of the metrics by proxy of being so poor last year, will be have to go up), but I think the "youth" (lack of track record with high snap counts), the turnover numbers I explained above, and some of these troubling metrics against the run/redzone, I am not sure how anyone can objectively say we are elite or where they absolutely say they see improvement from the defense.