Almost sounds like wishful thinking from a Chargers homer. It's like one of those Billy Beane trades where he gives up a few months of Mark Mulder to get a young Dan Haren, etc. The Chargers are a bit of a finesse offense that lives off a short yardage timing pass attack similar to the Patriots, and they play in the AFC, so it's a good spot for Mariota. The Chargers tend to run the football a lot when their RBs aren't injured, which also helps.
This trade only makes sense as a short term job security move for the powers that be in Tennessee. The Titans are one of the most talent depleted teams in the league. It would be a bit like the Erickson Seahawks bringing in Warren Moon. It will put them on the edge of the playoff hunt, but only for a few years and then it's time to start over again.
I would understand this rumor a lot more if it was Tampa. Tampa is bad, but they have a lot of young talent and could improve dramatically at any moment.