They're going to regret that next year, and probably this one. But I guess they thought it was worth the gamble. Not like they had a QB anyway.
I don't know if they'll regret it,
per se. Even with a more small-time arm at this point in his career, his smarts and ball protection will be a step forward from Winston's recklessness.
They don't pick high enough to pick one of the two obvious blue-chip QBs in the draft this year, so it's not like they're sabotaging their future by giving up the chance to marry Joe Burrow so they can have a one-night stand with a guy in his 40's. And they can still draft a QB in the middle rounds and let him watch and learn from Brady for a couple years, so it's not like they have to totally ignore the future just because they brought in an old-timer.
The only way they regret it, I think, is if they chose the wrong QB.
If Brady hits the wall and is legitimately bad in 2020 while Rivers thrives in Indy, they'd regret having chosen the wrong old man. If Bridgewater or Winston goes on to have a good career as a starter from here on out with with their new teams, maybe they regret picking the old man instead of being the team that made it work with younger guys. Especially if Bridgewater has a productive long era in Carolina where Tampa will have a front-row seat to watching it as a division opponent.
I'm not sure how likely any of those scenarios are.
I think the most likely is that Brady continues to get worse like he has been and he doesn't win a trophy. But none of the "road not taken" options pan out great enough to make them regret choosing Brady over them. They benefit from the boost in fan interest and media attention and relevance from being the team Brady finishes up with, and in 2 years, they're back to square one no worse for wear without having missed out on anything great.