I've thought about some successful programs who, in recent memory, have promoted one of their coordinators to head coach after the departure of the previous coach because that's what we did with Ryan Day. For the most part, that strategy has been successful. Here's a few examples:
Chris Petersen (Boise State)
Chip Kelly (Oregon)
Jimbo Fisher (Florida State)- I know his last season wasn't good, but he did lose his starting QB and have to use a true freshman. Plus, there was a lot of talk that there were tons of internal conflicts. He still won a national championship, went 6-2 in bowl games, and had an overall record of 78-17 before his disastrous last season
Dabo Swinney (Clemson)
David Shaw (Stanford)
*I didn't include Lincoln Riley because he's inherited 2 straight Heisman QBs. It'll be easier to see how good he is after the next couple of seasons or so. Mark Helfrich was good for 2 seasons with Mariota, then went downhill. Clay Helton was good for 2 seasons with Darnold, and just finished 5-7. Both of those examples are why I think it's too early to say Lincoln Riley is a success.
I love the idea of of trying to keep the continuity by hiring from within when you have the right guy, and Day seems like the right guy to me. He got 3 games of head coaching experience last season, so at least it isn't completely new to him. Plus he already took action to fix the aspects of the team that needed fixing. I also think he's inheriting more in terms of both talent and resources than any of the coaches I listed above did when they were hired. In my opinion, we're in store for a great 2019 season and a great future for years to come