I'll start with this... I am a Tannehill fan. So I want him to comeback next year and play at a high level and lead this team into a bright future. I don't want to see us move on from him. However, it is not impossible for a first round rookie to challenge or push Tannehill.
Let's rewind to the beginning of last season. We started the season 1-4 and during the game against the Titans some fans were calling for him to be benched and Matt Moore to come in. Through the first five games last season Tannehill had 6 TDs and 7 INTs, a pass completion percentage above 60 and a QB rating above 80. Now thankfully Gase backed Tannehill and we went on a run. During that run he had only one bad game against the Ravens, the rest of those games he played well to very well. But here's the thing about that situation, Matt Moore is a backup quarterback and fill in starter, he's not a first round pick. So would Gase have backed Tannehill if we had a first round quarterback holding the clipboard?
I don't think he would. It is one thing to brush off benching your starting quarterback for a career backup, but a totally different thing to do that when you've got a highly touted rookie on the books. This year Chicago benched Glennon ($15 million, multiple year deal free agent) for Trubisky (2nd overall pick). Last year the Rams benched Keenum for Goff (1st overall pick). A few years back Jacksonville benched Henne for Bortles. When your starter struggles and you've got a highly touted rookie quarterback on the sidelines, it is rare to see a team stick it out with the guy that is struggling. So obviously if we draft someone in the first round and Tannehill struggles early doors, a quarterback change is likely.
As for what a rookie can bring to training camp, well, I would tend to agree that it would be difficult for a rookie to outperform Ryan Tannehill, but it is not impossible for a rookie to challenge or push the starter. Last year Dak Prescott outperformed every other quarterback on the Dallas Cowboys - that included Tony Romo. This year the Chiefs drafted Patrick Mahomes tenth overall and he had a strong camp and some Chiefs fans actually wanted him to start over Alex Smith. Now admittedly, Prescott would not have been the starter had Romo not got hurt, and Mahomes is still the backup with the Chiefs, but both rookies left their mark and made a strong impression. One could argue that Mahomes' addition has brought out the best in Alex Smith because he knows he's got to play lights out otherwise Mahomes will take his place. Perhaps the same can happen with Ryan Tannehill if we add a highly priced rookie quarterback, but at the same time if Ryan Tannehill struggles with injury or performance on the field, then that could see Tannehill benched in favor of a rookie quarterback in year one.
So yes, rookie quarterbacks have come into this league and put immediate pressure on their team's starting quarterback, and while Tannehill has had a solid career to date, he has never been a top tier quarterback. What I mean by that is he's never gone on to join the likes of Aaron Rodgers, Tom Brady, and Drew Brees in that top tier. He's stayed in the same good, not great, category. I still believe he is someone you can win with, but when you are only a good quarterback your position is not as secure when your team brings someone in that can challenge and push you. For instance, Jimmy Garoppolo was never going to oust Tom Brady, however there is a very good chance that Jimmy Garoppolo could have ousted a number of other starting quarterbacks in this league had he been drafted by them. So while I am a Tannehill fan, I am also more than willing to admit that if our front office drafts someone early in the draft, he could end up being replaced if he doesn't play lights out football because he's not irreplaceable.