Shaw would only be used as CB if Martinez is injured and perhaps every now and then to rest him. He'll still play the vast majority of the season at leftback so it's not like we'd have to replace him there. It wouldn't be any different for him in that regard than it has been this season, where we bring in Malacia or Dalot to play leftback when Shaw moves inside.
We'd basically have four top players (Varane, Martinez, Kim and Shaw) to play throughout three positions, which considering they are in positions that normally don't see that much rotation is about as much as you can ask for. After that you are looking more at decent squad level players, so the question is whether we're better off with buying another cheap left footed centreback to cover Martinez while keeping Shaw at leftback, or bringing Shaw into the centre with Malacia/Dalot playing at leftback. The former would require signing another player (who would likely barely play considering Martinez would be expected to play the majority of games), while the latter wouldn't require any more signings.
The same holds true if we keep Lindelof and don't sign Kim or anyone else. While it might be nice to have top quality back-ups, it'd be hard to keep them happy (there's already been rumours this season that Lindelof wasn't happy having to share the back-up role with Maguire) and there are other positions that need that money spent on them far more.