It was more a case of the DL fans at TSR trying to make lemon out of lemonade. Fifth Age wasn't on anyone's radar when W&H wrote DoSF; they wanted to take the single-book contract and turn it into a trilogy or multi-book series with game supplements all focused around the Chaos War, but TSR said 'no, just the one book, please.' After presales of it were very strong, the DL fans in TSR's Games Department finally got the go-ahead to do a new DL game under the stipulations 'card-based, not-D&D based, and set after DoSF.'Even before I ever heard about the marketing reason behind Summer Flame, the book seemed like Weis and Hickman throwing all their toys against the wall to shatter them and then storming out to go home. The whole book felt like a capital middle finger. I don't know if they intended it as a middle finger to TSR, but it came across as a middle finger to the reader.
And THAT was the springboard for Fifth Age.
So despite what some people may say, DoSF wasn't written to introduce the Fifth Age. Weis & Hickman have gone back and forth on whether it was meant as a move into a new era or an outright ending to the series, and 25 years after the fact, I don't think we'll ever get a clear answer on that point. The War of Souls was more clearly intended as a course correction/launching point, although I pick up a lot of 'trash the Fifth Age and make sure no one else can ever play with our characters again' subtext in those … works … as well.