Lyxen
Great Old One
Well, the thing that would concern me is if they release 5.5/6E versions of the PHB & MM. If that would then make the originals unavailable in Beyond, I'd be mighty unhappy. From past experience, I haven't seen D&D's electronic fare continue to support retired product (other than PDF).
Again, I might be wrong, but I expect the changes to be minimal, at most at the "option" level of the current ruleset, certainly not what people would call a 5.5 or even a 6e. In which case, my concern would be about the amount of options, but hopefully the streamlining will also remove a number of those, hopefully the lesser used ones (and I hope that WotC's contract with DDB allows them to get analytics on those, they would be really useful in making the change, probably even more than a huge playtest, especially in terms of value for money).