Easy answer, they pull the trigger on a franchise QB at #1 in 2020.
Let's look at the alternatives.
1) Rosen balls out and conclusively determines that he's the man for the future … so we'll not get that #1 overall pick.
2) Rosen has positives and minuses but trends up and flashes great ability, enough to warrant another 1 year trial period. Well, if we don't draft a QB in 2020, the coaching staff is kinda wed to Rosen at that point, so if Rosen falls on his face in 2020, this coaching staff is likely replaced and some other coach pulls the trigger on the QB in 2021. Or, Grier gets replaced and the new GM brings in his guy. Either way, too much pressure on the coaching staff to string it out.
3) Rosen fails to convince. If we don't draft a QB then, the coaching staff will almost certainly be fired before the 2021 draft, and the new staff comes in with a TON of young talent and a top young QB from the 2021 draft to look absolutely fantastic compared to our current coaching staff. This is the proverbial worst case scenario for Coach Flo and staff, not to mention the new GM who replaces Grier if we don't draft a QB after Rosen flops.
4) Rosen flops but we receive a king's ransom for the top pick …. like maybe three 1sts and two 2nds. Well, it would be hard to say no to that, but there's still a 50/50 chance that this warchest of draft picks is spent by another coaching staff and GM, even if it is the right thing to do to take those picks in trade.
Each alternative seems to put a lot of pressure on the coaching staff and GM to draft a QB in 2020 if we get the #1 overall pick. The only scenario I see that doesn't is if Rosen does well, but then we'll not have the #1 overall pick unless it is the Texans who are the worst team in the league … and I think the odds are greater that we're the worst team.
If I'm a betting man, I'm all-in on the Dolphins drafting a QB in the 1st round in 2020.