Shows are big and expensive productions and the crew gets paid on off days (which is why there is only one day between shows on the current tour). Theres no sense in doing a tour of the United States, only hitting eight cities and taking long breaks between shows, it isn't financially viable for an act like Sabbath. If they did shows with Bill they would likely just be sporadic festival shows, and if thats all they're going to do then why bother? Better to go out on a high note with the 13 tour, or perhaps maybe a final massive farewell show with Bill.
I don't see any of it happening though. It'll either be another leg in the Spring or a break to relax and maybe work on more material.
One thing I'm noticing is that before the tour there was a lot of excitement with Ozzy and Tony fuelling speculation about making this reunion an ongoing thing, but now that we're a month in we're hearing from Geezer and Tony that this is very tiring. I think they underestimated how much strain a full tour would put on them and now that they're actually experiencing it they're starting to reconsider future plans. Weirdly enough it's Ozzy of all people who seems to be getting stronger as the tour goes on.