I don’t think they made it up, just didn’t end like people thought it would. I remember from day 1 they said they had the plan mapped out. If you remember there were filler episodes (the actors and the spiders) in I think season 3 and the show runners said that they didn’t want to do that anymore. They admitted that ABC made them make filler episodes to get to 24 per season. That’s when they said 3 more seasons, 16 episodes a piece. Abrams, Lindelof and Cuse have done a lot of other decent work so I tend to believe them that they had the end game in mind. Again, might not have been everyone’s favorite but I think making it up puts it in the same bucket as say Matrix 2 and 3, which were clearly, crap we made a lot what do we do next. We also didn’t have creative differences and change showrunners like TWD and watch it become meh. I loved the early stuff more than the ending but still liked it all.