So many ways they could have done this if they wanted, they could easily delay the main story for years and we+hbo would gladly wait,they just wanted to move on to other projects so they decided to wrap it up. It is what it is.
Jon could have sat Bran down and said. Tell me everything you have seen. You then do a flashback and show us the entirety of Robert's Rebellion, stretch it out to a whole season. Rheger, Lianna, the trident, everything. Would have been fantastic. Id pay to see that. Then when the books finnally come out snap back to the main story and continue with the piffery. This way you still get to do a sort of spinoff and still keep the regular story going with the traditional slow pacing and nuance of the show we all fell in love with because you have guidance from the Author.