I've never really been a big believer in the theory that D&D rushed the ending to go and make Star Wars. By all accounts, the decision to shorten seasons 7 & 8 was made in 2015, even before The Force Awakens was released - so way before any deal with Disney was even thought of, let alone announced. Plus, it took them nearly three years to make those thirteen episodes - I'm not sure they'd have put in so many hours if they were eager to cut the show loose and do other things. I'm also doubtful that the show "could have run for 12 or 13 seasons" like so many people, including GRRM, claim. At that stage, I think you do start to lose viewers and even lose creative control. I think from the beginning they aimed for a "seven books, seven seasons, seventy episodes" structure, and they aimed to stick to that.
But that doesn't necessarily mean they shouldn't have changed their plan. GRRM originally wanted the ASOIAF series to be a trilogy but, like all creative people, things changed for him as he went along. He might be in a muddle now as a result, but at the time it was absolutely the right decision for him to expand the series and use as much space as he could. At the end of season 6, I think the general consensus was that D&D had cut a few corners (like wiping out the Tyrells and Dorne, and not including certain plots from the books) but that they'd done so economically to keep things exciting. The best and most successful serialised shows tend to stop expanding after the halfway point - the story becomes more centrally focused and psychologically driven (as opposed to sociologically) - and so they understandably followed suit. If stories were like real life, they'd be very boring and they'd never end.
But I think season 7 (the weakest season by quite a way, imo) nearly buckles because it doesn't take the time to open up the final act of the story. It speeds up when it should briefly slow down. Remember how long it took for the first major plot explosion in season 1? It simmers away for 6/7 episodes and then
bang, Ned's arrested and beheaded, Robb's off to war, and Daenerys has three dragons. Then think about how long it takes for things to happen in season 5 (the underrated start of act two) - it simmers away for 6/7 episodes and then
bang, the White Walkers attack Hardhome, Stannis' campaign collapses, Cersei does her walk, and Jon's dead. But then think about season 7. It has just as much responsibility to open the final act, but it stuffs a number of major landmarks in right from the get go. Don't get me wrong, I still find it entertaining and exciting, but by the time our favourites are heading out beyond the Wall I'd not really had the logistics explained. Season 8 has similar issues, just nowhere near as severely (only in my view, I must stress).
Now, why did they do that? As I said, I don't subscribe to the theory that they rushed it to bugger off (Liam Cunningham was quoted at the time saying "Usually we do 10 episodes in 6 months, at the moment we're doing 6 episodes in 10 months"), but my own theory is that they either lost confidence, or interest, in working through the finer details. I think the overwhelmingly positive reaction to 'The Rains of Castamere' or 'The Watchers on the Wall', and especially 'Hardhome' and 'Battle of the Bastards', gave them the belief that they could put all their eggs in the spectacle basket and connect them with bullet points. Now, I thought the bullet points were compelling enough and added up to a conclusion that made sense, so I've never really had much of a problem. And if you break it down to minutes-per-season, season 8 is only 120 minutes (so, two episodes rather than four) shorter than seasons 1-6. But I think, either because they were sat too close or because nobody told them otherwise, I'm not sure they gave enough credit to how much
story was actually left.
As we all know, there's a very delicate balance between plot and story, and the last two seasons really walk that line. Season 7 gets it really wrong in some places (even objectively, I don't think season 8 has anything as bad as Arya & Sansa's little feud), and you can't ignore the writing gears working hard in the background to force things into place for the final leg (I don't think season 8 has anything as "WTF?"
as them going beyond the Wall). But I don't think it comes from them thinking "Ah feck it, let's toss this off and get out of here", because it took them a year past their personal schedule. As showrunners and writers I think they just knew they were brilliant at delivering the big moments (critics, super-fans, and all their awards told them they were), so they just relied on that. Even the show's weaker plotlines were rescued, and even enhanced, by outstanding climaxes, so I don't blame them for thinking "Ah, this is what people want!" when it came to the very end. I think they made the wrong choice but I'm not disappointed in them, or mad at them, for thinking that a condensed story was the way to go.