Yeah I suppose so. He could still back-up Kingery and Segura in a strict platoon but I guess you'd rather have Quinn play CF against LHP if they're still looking to limit Haseley's exposure there. The question there really becomes who is worse, Miller or Haseley (but we don't really have any data on Haseley and in the long run you certainly hope he can play every day).
In theory Miller and Harrison could both be on this team though. Miller a LHB who can play SS. Harrison a RHB. They can both play OF corners. Plus Bruce (lefty bat, LF/1B/DH), Knapp (or Grullion), Quinn (the 26th man if bullpen remains 8). Plenty of double-switch options against relievers. Not sure they're gonna find a better extra OF and a glove-first, SS-specific guy still seems unnecessary.
It seems unlikely they would pursue anyone better/more expensive than Miller for the bench, though there are plenty of people they could sign instead. Those millions will go to pitching. Ditto catcher, there's no $2 million guy out there who would be that much better than Knapp when they need pitching. Also I assume if Knapp were to be optioned (as some have speculated) his $700K major league salary would still count against the cap, as he's past rookie minimum/into arbitration-eligibility. If he's not the guy you trade or cut him.
It's all or nothing, IMO. Sign or trade for a 3B or CF who also considerably strengthens your bench and the other position (by putting one of Kingery or Haseley on it most days) or this is it.