For a couple reasons. First, he might get snapped up by another team with a lesser tender. Secondly because if he's going to be starting at LT $6M should be no problem for the team to swing. I'm sure he'd appreciate that extra couple million bucks too.
And perhaps most importantly playing him on the RFA tag for 2024 allows you to get more evaluation time on him before you make the decision on whether to pay him as a LT. Because that is serious bank. You can't afford to miss on that. Rams already did a halfway signing job at LT trying to hit on Boom in that role on the cheap (for LT) and it burned them. They have no problem paying guys who deserve it, but I think they need to be smart here.
I'm not sure he's the guy you want to put your trust in on that blindside. It's a tough job, and a critical job for the QB's mental outlook. With Stafford him trusting that LT can be the difference in him being disciplined or him resorting to bad habits.
Lastly IF they don't get Dotson signed then I think the plan will be to move Jackson to RG and draft a LT of the future in round 1.
