Where to start. I would think that after spending the last 3-4 years developing content for Destiny 1, learning what people liked, what they didn't like, how people played and engaged the raid content, improvising, modifying and adapting their tools and technology, increasing resources and manpower across the board and improving their content generation pipeline, dropping last-gen console development (which we know caused significant issues with previous Raid encounters) etc... Bungie would be able to produce more higher quality content faster than before. Plus they went out of their way pre-release to talk about how one of their main goals with Destiny 2 was to not only to deliver a great game but to also deliver more higher quality content faster than before.
The result of this is Raid lairs.
Between Bungie, High Moon Studios, Vicarious Visions and Activision there are over 1,000 people working on the Destiny franchise, I don't think it's out of the realm of reality to think there's enough resources on tap to make new Raids with each expansion.
The fact that Destiny 2 comes with one raid, and that each expansion for two years in a row has only come with one raid, should be evidence enough that making a raid is a huge undertaking. More importantly, it's one that needs a unified vision and direction. I'd say very little of game development can be consistently solved by throwing more and more people at the problem, otherwise all of the Assassin's Creed games would be masterpieces. There is a raid team, but they probably design mechanics. There is no way that multiple devs are being forced to cooperate across multiple time zones just to create new raids. Quite frankly, that's not worth the investment.
On top of that Bungie having a better understanding of what Destiny is this time around, and what type of content players are expecting would be planning each piece of DLC and Expansion well in advance this time around. TDB was a mess because it was rebooted and scrapped together in 9 weeks, that sucks and but I expect them to learn from their mistakes and actively try and do better.
Raid Lairs sounds exactly like the culmination of this.
Now we find out that Curse of Osiris didn't even start full-on development until after Destiny 2 launched.
That does not mean it was not planned like this. Do you really think they had a Mercury raid ready, and then scrapped it?