I'd put the end of the road for DBS somewhere around that decade mark, i.e. 2028. But it'll be a pretty bleak, long tail. Seeing rooftop dishes or knowing anyone who uses one will be a pretty rare thing by 2025. It certainly won't make sense to ever build and launch new replacement satellites (after, perhaps, the one you claim that AT&T will launch soon). As more and more Americans, including rural dwellers, gain access to broadband and OTT-delivered video alternatives (e.g. T-Mobile TV via long-range 600 MHz LTE/5G, as well as a cheaper "full DirecTV" via OTT), you'll see the numbers on DBS rapidly dwindle, increasing the per-sub costs for installing, operating and supporting the service. And during that time frame, I think you're going to see the continued erosion of the overall linear channel TV model too, as more and more viewers migrate to services like Netflix, Hulu, Disney's forthcoming OTT service, HBO (or whatever it morphs into as it becomes a portal for various AT&T-owned content), etc. Once a broad variety of live sports become available via standalone OTT services apart from traditional linear channel bundles, that's all she wrote. So DBS will be a shrinking slice of a shrinking pie.
The more interesting question, I think, is whether AT&T holds onto their DBS all the way until it dies or can they find a buyer for it in a few years? Dish and DTV use different satellites but there would be some sort of economy of scale, I would think, in a single company operating all of them -- one set of channel contracts, one set of in-home STBs, zero direct competitors. (Of course, for the government to approve such a merger, one or both operations would likely have to be in dire straits.)
I'm not convinced that Dish can survive another five years on their own. Sling TV isn't going to save them. Ergen is going to spend a little money to build out a fig-leaf of a 5G IoT network so that they can keep squatting on their spectrum in hopes that someone buys them out for it. But I don't see that happening. The last FCC auction to sell TV spectrum for refarming into cellular didn't fetch the kind of prices that many had anticipated. With the advent of 5G, plus low-earth orbit satellite internet (SpaceX, etc.), plus the expansion of fiber and symmetrical D3.1 cable internet (not to mention the opening up of various bits of unlicensed spectrum and whatever other technologies, such as AT&T's AirGig, end up bearing fruit), I don't see there being a huge hunger for Dish's spectrum to serve as additional last-mile pipes to consumers.