Makes sense. If we assume better health for Hayward and Irving next season (and, I mean, if they don't combine to play more than 61 games total Boston has bigger problems than the fit of their 3rd/4th point guard) you ideally want your backup guards to be able to play off the ball as much as on, and Wannamaker has the frame to hold up much better defensively than Larkin (who, don't get me wrong, was very good relative to his role/salary last year). He doesn't need to be a young guy with star potential, he just needs to be a veteran in the locker room who doesn't panic if he actually winds up playing minutes that matter.
That could relegating the Celtics to re-signing Baynes using the 120% bump they can give him using his Non-Bird rights.
If Baynes returns next season it'll almost certainly be using his 120% Non-Bird Rights. His Non-Bird amount is functionally identical to the Non-Tax Mid-Level next season, and I can't imagine Ainge hard-capping himself to round out a four-man center platoon on a team that will primarily play small. I honestly wouldn't be surprised if Williams and Wannamaker are plugged into the roster spots of the departing Larkin and Monroe to remove some of the defensive liabilities from last seasons' roster, Smart and Baynes return, and the biggest summer controversy for the C's is whether Nader or Jabari Bird get the final full roster spot going into the 18-19 season.
It would have the added benefit of keeping the Non-Tax Midlevel available to use on buyout guys down the road (even though it gets pro-rated, unlike the DPE from last season).