While there may be no technical reason like we had in MoP with the 4 billion Boss HP maximum, due to the exponential growth we get some completely different problems sooner rather then later.
The main example would simply be balancing over the whole expansion. Yes, you can have a road map, but at some point these 15% more power or whatever between tiers will be so large that you can start to do the previous raid in really small groups with only a few pieces of gear from the current tier. I mean, you don't have to look any further than Mione being able to solo 10 normal Gul'dan, the endboss of the last tier. And we are not even at the end of the expansion, a whole tier is still going to happen. I wouldn't be suprised if all of Nighthold could be soloed by some people at the end of the current expansion.
As for me, I am also for a level squish as well as a stat squish. The level squish would even accompy it, simply by removing the stats you would gain from regular level ups from the then missing levels. Additionally, it would allow Blizzard to pace out new abilities better, so we won't have gaps of 10+ levels without gaining a single new ability.