No, picking a quarterback in round one doesn't guarantee that they will be a franchise quarterback. However, it certainly takes more than one season to determine if they are a bust or not. Heck, the fact that Dolphins fans are even discussing moving away from Tannehill and drafting a new QB in the 1st round before even one training camp practice shows how impatient Dolphins fans are. The fact is that Tannehill might not play much at all this season. Maybe he gets a few starts at the end of the season, but unless he starts day one you can't make a final determination whether he will be a success in the NFL or not. Even a full 16 game rookie season is not enough. There is no way to make a decision on Tannehill's long term future based on one season.
Also, what guarantee that Barkley is a "bonafide franchise QB"? How do you know that for sure? You don't, Tannehill may turn out to be the better pro. There is no guarantee that Andrew Luck as the #1 pick is going to work out for the Colts. Remember guys like Ryan Leaf and Jamarcus Russell? You don't throw away one unproven commodity to pick another. Like I said unless the front office and coaching staff are completely gutted absolutely no chance, nada, zilch that Barkley would be picked by the Dolphins if he is on the board. Miami would either take the best player(not a QB) on their draft board or trade the pick to a QB needy team for multiple first round picks like the Rams did this year. Sam Bradford has already been in the league two years and hasn't exactly proven himself, but the Rams are continuing to build around him and allowing him to develop even though RGIII was staring them in the face. You want to get a franchise QB and instill confidence in your player and organization? You don't undermine your first round QB by taking another first round QB just a year later, before allowing him the time to prove himself. You either got it or you don't as a rookie? Matthew Stafford sucked his rookie season and was invisible his second season. His third season he became a star in the NFL and is now one of the top quarterbacks in the game.
Do you know the last time a team took a quaterback in the first round of the regular draft in back to back years? You would have to go all the way back to 1982 and 1983. That was a unique situation though because the quarterback taken in 1982 had been suspended for the 83' season. Before that, you have to go all the way back to 1962. As you can see, what you are suggesting never really happens, and it won't next year.