Well, he was their HC from 2019-2011, so they'd be familiar with him. I can't think I've ever heard anyone say a single negative thing about him personally, and most people seem to respect him. But if Jeff Fisher is Mr. 7-9, Caldwell may be Mr. 9-7.
I think the real issue though is that at 63 years of age, there aren't a lot of coaches that want to take that on. Jimmy Johnson said that Belicheck, Pete Carroll (high energy) are anomalies, and it's really a young man's game, with long hours and a lot of stress. It's just really hard to find a Sean McVey.
Having said all that, they could certainly hire someone a lot worse.
As to Luck, we will have to wait and see what the truth is on his injury. He may heal, or may be the next Chad Pennington. If he can throw though, and move okay, he may have to turn into a passer who releases a lot quicker than the past, and simply doesn't risk getting hit or sacked if at all possible. Or just takes a quick fall to the ground when it's close. Think: Tom Brady, Peyton Manning's last few years, etc. It can be done, and he could have a successful career at it. But if he's not completely healthy and strong, he likely can't extend plays the way he could, the way Ben Rothlesberger or Cam Newton can, let alone run like Cam still does (and Luck sometimes did in the past). And Luck is going to have to have an open, progressive mind about this. Just look at RG3. Again though, I'm not his doctor, I'm not a doctor at all. He may be back to 100% health come next season. I doubt the Colts will draft a QB though, as they have Brissett, and he played fairly well, with room to improve. Who knows though? They do have the third pick in the draft, it may tell us something about the team's faith in Luck's future.
In fact, it's the top story on NFL.com's 10 stories for next season to watch:
http://www.nfl.com/news/story/0ap3000000915618/article/ten-offseason-storylines-that-will-shape-the-2018-nfl-season
Garrapollo signed the biggest contract in NFL history, maybe. Ink not dry just yet, but it looks like it. With all SF's cap space, I won't be surprised to learn in 2018 his salary will top $40m in a front loaded contract like that.