Reason for optimism: There's lots of stability from the team that won 86 games last season.
Reason for pessimism: It's really not that different from the team that won 74 games in 2015; that team is just two years older.
If you strip away the luck involved in converting team production into both runs scored and allowed, and remove the luck converting a team's run differential into wins, then the Tigers deserved to be playing in the AL wild-card game last year. That performance marked an impressive bounce-back from the last-place finish in the AL Central in 2015. Their results last year may have been earned -- and it's too bad it didn't result in a playoff appearance -- but mainly due to the effects of aging, it doesn't look to be repeatable.
However, Detroit's management took a look at the hand it's been dealt and announced: I'll play these. It's possible that every pitcher who started a game for the Tigers last year -- nine total -- will be on the Opening Day roster. That's a virtually unheard of stability in today's MLB environment.
The story is very similar for the every-day players. While some (unexciting) combination of Tyler Collins, Mikie Mahtook, and JaCoby Jones will combine by season's end to take the bulk of the starts in center field, the other eight lineup spots are expected to be manned by the same players who had the bulk of those at-bats last year. Again, you just don't see that on other rosters.