I think this is a bit like that saying “people always prepare to fight the previous war”. 343 clearly looked at Halo 4 and said. “Right, a ton of people bought the game at launch but the multiplayer population rapidly dropped off. So, we’ll have these big multiplayer updates and over a year of post game support to keep the people who buy the game this time.”
The problem was that unlike Halo 4 they didn’t have a ton of people buy the game at launch and so they put a lot of effort into maintaining a smaller playerbase. Whether that was because of the campaign reviews, stripping the multiplayer to barebones or the competition I don’t know and can’t say. But I think this was the rationale underpinning why 343 did what they did. Clearly they assumed people were invested in Halo 4’s campaign so they would naturally buy Halo 5. Think about it, if your campaign is well received but people stop playing multi quite quickly you’ll probably assume that people are buying your game for the campaign and Halo 5’s marketing was very single player focused.
So I think in Halo 6 they’re going to look closely at Halo 5 and we’re going to see a strategy that responds to what happened. They’ll prepare to fight the previous war. So I think we’ll see a big push to get people to buy the game at launch. Loads of advertising and hopefully stressing how much stuff is going to be available at launch. I mean they’ve arguably already started advertising for Halo 6 with stuff on this sight, comics, possibly even HW2 since its set post Halo 5. But I think we won’t see as much or as thorough post launch content. Possibly they might switch pack to more traditional paid DLC maps and a Season Pass instead of all of the free updates.
We might also see them switch from the single player focused advertisement we got in H5 to really pushing the games multiplayer component as why you should buy the game. Although, in practice, game devs like to use the campaign in advertising even when its a small element; ie Battlefield 1. Its just more compelling and makes better entertainment. But it follows that if Halo 5’s campaign was despised in so many quarters and its multiplayer was praised that they will focus on that more this time around.