Well, NBC Sports Bay Area 49ers insider Matt Maiocco asked Kyle Shanahan that very question during his press conference Monday. The 49ers coach's answer was as lengthy as it was revealing.
"I think everyone's [response] is the same after it happened. ‘Why didn't you just get out of bounds?' " Shanahan said. "I think it's a reminder to all quarterbacks and coaches. Just turn on the tape any Sunday, and you're going to watch 15 plays by quarterbacks jumping over people, not getting out of bounds, doing a little extra to move the chains where it's a hell of a play and everyone's glad they did it. Then, you see some guys get out of bounds, and you're like, ‘Yeah, that's what you're supposed to do.'
"Then, you have something like this that you're reminded of why you do coach that and why quarterbacks should do it. But, those guys are competitive guys who want to do whatever they can to win the game. You're not assuming just by going and staying in bounds you're going to tear your knee up. Sometimes, you get hit a lot harder because of that, which is what I thought happened until I heard later.
"But, I think that's something that Jimmy will probably look at differently going forward because I know he'll remember this the rest of his life. Nothing against him. I think this happens with everyone. Like I said, you see it every Sunday.
"I think it's just a reminder for everyone why that is the obvious coaching point and why you need to stick with that."
Shanahan is right that Garoppolo won't soon forget his decision. It cost him the remainder of this season, and now he'll have to look forward to rehab and 2019.
"He's down, just as everyone would expect," Shanahan said. "Jimmy was real excited about this year, and we were real excited to go through it with him. Got to go through a few games, but I know he was very disappointed. I know it was hard for him last night, it was hard today, and it'll be hard over these next few weeks.
"But, Jimmy is a tough person. He'll bounce back from this, and he'll be ready to go next year."
NFL coaches likely will show their quarterbacks video of the play, hoping to avoid the same fate the 49ers now face. Shanahan, though, is confident in what Garoppolo will be when he returns next season and what C.J. Beathard can be in the interim.
"Quarterbacks can come back from knee injuries. They do it all the time," Shanahan said. "I don't think this will affect him going forward. Yeah, he'll have a long recovery. He'll have to heal. But, right when the offseason starts, I think he'll be there. ...
"So, I'm still very excited about Jimmy and the future of him here, and I feel fortunate that we have a backup quarterback that we have a lot of confidence in."