How much Rashawn Slater tape have you watched? Most of us have spent no time breaking down the OL prospects. I don't know how good Slater can be, but I know he did a great job against Chase Young last year, and Young has spent most of this season dominating NFL linemen. There are a number of people in and around the NFL who think Slater is better than Sewell. We'll see how the pre-draft process shakes out. Sewell seems great, and a lot of people think very highly of him, but he's not universally seen as the slam dunk generation talent that some on here see him as. Everybody thinks he'll be good, but just that there's not necessarily a massive gap between him and some other tackles.
The problem with trying to trade up to get Sewell is that you're going to be competing against teams that want to trade up for a QB, and so those offers will be steep. The Bengals offer would have to be overwhelming, and it's not worth that if the guy available at #5 has a chance to be nearly as good.