Most new players like the main story, it helps keep their leveling first time around enjoyable and the way it unlocks content helps them learn about the game in a gradual manner. I get the impression it is just some veteran players annoyed at this and not new players who dislike the idea because the veterans players might have done the main story so long ago that they don't remember main story has always unlocked a lot of content in this game, perhaps the veterans players spent so much time leveling alt classes on same character grinding fates, raids, myths, soldiery, poetics and more instead which has tainted their view on the main story progression method for first time players.
Lets not forget the Umbral Era of the MSQ help level you from 1-50 so those are not complaint worthy in my opinion given they help the new player learn about the game and level their first character to 50 while keeping it interesting. Also take into account the MSQ from the expansion will continue to help them level from 50-60 while keeping it interesting so can rule those out as complaint worthy as far as I am concerned. The only possible complaint worthy potential thing is the Astral Era MSQ but when take into account those unlock a lot of interesting content, most new players won't mind them I am certain.
When think of it from a 'first character' in the game perspective not tainted by veteran bias, I bet almost no new players will care that they have to continue the main story to unlock the level 50+ expansion zones since almost every few main story quests unlocked something new from features to content and they would probably view this as the same as they are used to doing. New players tend to consider the MSQ a blessing and not a curse so more of it to unlock more content is not something most would rage against. Maybe the following expansion MSQ will help people level from 60-70, again that would invalidate any concern in my opinion for those ones given they would make leveling from 60-70 more interesting.
I doubt many players would expect to waltz right into Ishgard at level 1 just because bought the expansion, they still would have to level from 1-50 (expansion zones are 50+) and they would still need to learn about the world they are playing in and unlock things in a gradual manner so not to become overwhelmed. Umbral Era quests help them get to 50, Astral Era unlocks a lot of interesting content and the expansion MSQ will help them level from 50-60. Honestly people complaining about this are making mountains from molehills in my opinion and as someone who has just had their friend start playing this week so know what a new player thinks about it from a new player perspective, the new player friend has no problem at all with this news.