I'm actually surprised there haven't been more complaints about the linearity after FFXIII's release (because everyone and their dogs were bashing it for how linear the maps were, and that you could see how linear it was on the minimap).
Funny how people didn't seem to think FF7 was linear, although it only gave you one path to progress. You just had the option to backtrack (but no real reason to for 90% of the game). Fort Condor was optional and could be ignored, and 2 characters could also be ignored or never gotten. But the entire path westward to rocket city and then eastward to Temple of the Ancients, are linear. By the time you need to find your (only way) to Icicle place, it's the only place you haven't visited, besides possibly Wutai (but I'd do it later, with a flying airship). And people didn't pan it for being linear.

My guess, the time WoW launched
When they went from openworld dungeons everyone can compete in to instanced dungeons where everyone has their own private things and where griefing/camping is harder to do.