Not that I totally disagree with your logic, but it's worth calling out that free agency decisions are made months before the draft. It's sound logic to say you're going to let Onwenu walk to replace him with a draft pick. But that means a draft pick that can't be used on a talented player (that we like more) at another position. Also, it's tough for me specifically plan on drafting a starting caliber player at a specific position in round 3 like you have here. Our 3rd rounder is #68 overall and OT is a premium position. You can't just assume that all of the guys we view as day 1 OT starters won't be snatched up in the top 67.
Also, $20M/year would put Onwenu at the top of the guard market (currently $19M/year). I think as a RG he is worth a top of the market contract. Our OL play has been absolutely terrible in recent years and I think it's important to have good play there to develop other offensive players. It's a big puzzle and it depends on what their draft board looks like, how they rank other FA OTs, what else they could get with the money being considered for Onwenu, etc. But in general, I'm in favor of bringing Onwenu back. Not sure he's a $20M/year player at RT but it's nice to have a good option there which he is. Ideally, I'd like to eventually move him back to RG but it'd be nice to not go into the draft absolutely needing a RT so our new QB isn't dead in the water.
