I would love to see more GoT after the main series is over, but they just can't focus on the big battles or stories of the past.
Part of what make people like GoT is in the uncertainty of what will happen next. "Will that character die?" "What will happen when he reach that fortress?".
Doing a prequel spinoff of the Mad King or Robert Rebellion would kill all tension or suspense because you already know who lives and who dies.
Eh, this is something that is always thrown around in regards to prequel materials (in part because the very word prequel is most famously associated with bad films), but it's not really true.
On one hand, there's nothing stopping them from doing a rebellion focused story that gives a lot of attention to characters who aren't Robert and Eddard. It'd be a cop out sure, but it's not like the existing series doesn't already give attention to a massive cast of characters.
One the other hand, the Dunk & Egg stories being mentioned here are explicitly about characters who die in a tragic event later in their lives. And that's what makes them even more compelling, because you have these stories about young characters you
know are doomed in a very big way. There's no reason that same framing can't work for a Robert and Eddard focused prequel about the Rebellion. A story about Robert's Rebellion becomes even more depressing to see play out because you know what follows.