Jessie Bates is the only major free agent that walks
The true best, best-case scenario would involve no big-name free agents -- including Jessie Bates -- leaving, however at this point that simply feels inevitable. Plus it would make the drafting of Dax Hill entirely pointless since he was very obviously drafted as a replacement at safety for Bates (or Vonn Bell, but I'm pretty sure they were anticipating Bates departing more than Bell).
The fact of the matter is Bates is going to be a free agent this year. The Bengals are not going to pay him the money he deserves as they need it for upcoming contracts (like Burrow's and Chase's, for example), and it's highly likely that he'll walk. The focus now shifts to, who will they keep?
Some big upcoming free agents include Germaine Pratt, Vonn Bell, Hayden Hurst, Eli Apple, and Samaje Perine. Hopefully, they're able to keep all of these guys but that isn't a guarantee.
According to Spotrac, Apple is going to be the most expensive to sign of these players at over $11 million per year-- which, no hate to Apple, but that almost seems like a typo-- with Bell a close second at $10.6 million estimated annual salary.
With how much cap those two alone could potentially take up, plus a $9.7 annual salary for Bell, this is really the most unrealistic of the three scenarios I've presented. Honestly, for how much some of them are projected to get, it might just be better for them to walk instead.
Still, you want to try and mostly retain a defense that has been nothing short of fantastic for the past two seasons.
Unfortunately, you will probably be seeing a lot of Bengals you've grown to love the past few years leave for other squads. One can dream, though...