You said "if he busts." I assume you were making that call after year 1.
In reality there's a couple scenarios I could see happen.
1: Best case - Watson plays well, shows progress throughout the year, coaching staff is pleased with his grasp of the offense, Bills go anywhere from 6-10 to 9-7. Would still take a QB in the first next season
2: Most likely - Watson has his share of gaffes, bad games, good games. Serious questions remain about his abilities in the NFL. Bills go anywhere from 7-9 to 4-12. Would still take a QB in the first next season
3: Worst case - Watson is terrible, can't play at this level, throws picks left and right, doubts he'll ever put it all together. Bills go anywhere from 6-10 to 2-14. Would still take a QB in the first next season.
4: Injuries - Watson plays around the #2 scenario but only plays 6-10 games due to injuries, or heaven forbid, not more than a game or two. Would still take a QB in the first next season.
The bottom line is exchange any of the Bills 1st rounders in the last decade with a franchise QB and we make the playoffs. Gilmore, Dareus, Watkins, Shaq versus Luck, Cam, Carr/Bridgewater, Wentz/Dak is a no brainer.
Season #2: Open competition between Watson and 2018 rookie QB. Whoever wins plays. Then at the end of that year, you'll know if Watson is the guy or not and already have a backup in the wings. How much better off would the Bills have been to have Carr waiting behind EJ when we had to yank him in 2014?
Guys like Dareus and co are great pieces and nice to have, but they aren't getting you to the playoffs, let alone winning you a Superbowl without a QB.