I mean i don't think anyone in any thread is denying the forums will be filled with players that will complain about this (some already are) just whether or not those complaints are justified.
Even IF a new player was able to access the new areas without a MSQ gate those areas (which are likely 50+) would still require you to level to 50 anyway to do anything meaningful in.
These players would very soon come to the realization that a large portion of the game's content is gated behind the main scenario in some way or form anyway. Large pieces of content from airships, to grand company access, to chocobo licenses and primals, even the abilities of certain jobs (See: Summoner) require you to have made progress through the main scenario in order to unlock parts of the game.
It's a far cry from fanboyism to expect people who buy and pay a subscription for a game to actually have to PLAY the game to experience the content...
Also the expansion and the base game are still sold separately it's not like a new player to the game is FORCED to buy the expansion out of the gate to play, however the game is making it very clear that IF they wish to play through the new content / jobs / areas that a certain level of progression from the original game is required.