Considering the amount of new rides and attractions we are getting in the coming years, I actually don't consider any of these timelines to be unreasonably long. For those of us who haven't experienced Pandora yet, between now and the end of 2019 (2 and a half years), we will get to experience 3 new themed lands and 10 new rides/attractions. I'm not counting net new attractions, simply new experiences. I can't remember ever getting that many new rides at once. MAYBE a 2.5 year window around when AK opened?
MK - Stitch Replacement
EP - Rat and something in the UK
HS - 2 Star Wars Rides, 2 Toy Story Rides, Mickey Ride
AK - 2 Pandora rides
Also possible in that time frame, Ellen replacement, another new attraction in Tomorrowland, and I would bet at least one more new ride in each park by the 50th. That's 16 rides in 4 years. And most of the ones we know about so far should be open in the next 2.