I don't think it signals too much about 2018, if anything a new manager is under less pressure, especially as he also breaks in three or four new starters and the rotation adjusts to a new pitching coach. They need to make a run at .500 but they don't have to be in the playoff mix (though the two do overlap, these days).
Honestly I think this was always the plan, or they wouldn't have let Mackanin start the season w/o the 2018 extension. Then the Benoit thing kind of forced Klentak's hand. I don't know that Klentak has someone in mind but I do think that when he took the job and kept Mackanin he certainly knew he'd be hiring his own manager by 2019 at the latest.
If all the off-season signings had worked out better and the team had won 81 games this year, you would have had to let Mackanin see it through for one more season.
My guess is all the coaches are gone except for Morandini and Stairs. Maybe Bowa goes back to 3B but at his age he might be willing to shift into a Clearwater/part-time gig. Samuel will leave if he's not the manager (and he won't be). Wathan could be promoted too (and Morandini could also be sent back to manage Reading or LV).
I think this is different from Charlie, Mackanin obviously has a lot of knowledge and opinions about the current 25 and 40-man roster that can be useful to Klentak.Not that Charlie doesn't bring something to the table because I think he does but he's also around for sentimental reasons, Middleton is not immune to that.