I can't agree to this statement.
when I play I don't usually build ringworlds(don't like them) and only build lots of habitats if i'm forcing myself to be peaceful. for me the true endgame perk is Colossus. cuz it opens total war.
and out of other megastructures most of them can be substituted with something else: mega-art - useless. mega-coordination centre - useless. mega-sensor - useless. science nexus = 1-2 science habitats. dyson sphere = trade ecumenopolis. only matter decompressor is hard to substitute, but you need a black hole (though you can try to build lots of forge worlds if you're able)
The Art-gallery is doubtful whether it's a good megastructure or not. However, the Coordination Center is very powerful because it not only makes your fleet bigger, but also makes your starbases a LOT stronger. Dyson sphere gives also a lot more energy than what trade even would give you, and it does so without the cost of other resources (I.E., you could convert that right away into more alloys/month. (Also, don't forget that the Dyson Sphere/Matter Disintegrator actually DOES work with edicts, unlike trade would), so most consistently a dyson sphere would give 1250/month.
The science nexus used to be great though. It's fallen behind a lot IMO, but that's because a lot of people abused this, as Tall empires became the meta. Pre-2.2, it was a common strategy to rush your way to a science nexus and complete it, then make your second.
Undeniably the mega-structures used to be godlike in lategame. In 2.2, it became a lot less appealing with the new Alloys system, as well as some being nerfed. Though, as again, they are still a core pick for anyone that's wanting to invest into the late-game.
Or you could just conquer the galaxy before you have time to build a mega-structure.
Read below, please.
I see a lot of comments saying "I win the game before megastructures." I'd just want to bring in my two cents here that this is not a valid agrument to say whether Megastructures should be guaranteed-content or locked behind a perk. Megastructures is a lategame feature. All the power to you if you finish the game before the megastructures point, but IMO a "I don't use it" isn't a constructive/valiable answer to a "Yes/No" question.