Dallas can probably putter along for a bit even without Hagman as a moderately popular soap by cable standards, and former fans may give it a ratings boost in S2 as they tune in to see how the writers will give JR an appropriate sendoff, but it'll never be the same.
The younger characters just don't have the oomph to carry the show on their own. The ratings will most likely spiral down and they'll be lucky to eke out three cable length seasons.