If I remember correctly, the shutdown of the Sirius side turned out to be more rumor than fact, granted, a widely spread rumor that I believed at the time as did plenty of others.
They are letting the Sirius satellites finish up their service and not replacing them in their polar orbits. All of the new sats are going to be geosynchronous which I think is unfortunate. I'd have much preferred that they keep both because geosynchronous works well for home receivers while the polar orbit works really well for mobile receivers.
They could, theoretically, double their bandwidth by replacing all of the Sirius radios with new radios that receive from both spectrums (XM & Sirius) and then just transmit XM on the normal XM spectrum and something like "XM+" on the old Sirius spectrum (same codec as XM - just on the old Sirius Spectrum). Then they could offer new radios that could see both Spectrums, understand that some channels come from "XM" and others come from "Sirius" and use the same codec for both.
I don't see them doing that.
As it is, and I think someone mentioned this above, you're still seeing Sirius tuners in new cars. You can still buy Sirius PNP units at the stores (somewhere in the back in the clearance section next to the XM radios - it's funny how they're no longer front and center).
My guess is that they'll just keep the status quo for the foreseeable future. They have their subscribers. They'll just (mostly) simulcast across the two spectrums.