I don't mind GAAS, if it's implemented correctly. There's nothing inherently bad with the concept. I found The Division 2 to be a pretty great implementation of it, and I found Sea of Thieves to be a pretty great implementation of it. But, neither of those have strong narrative experiences, and they're all about that bump and grind.
For me, it comes down to: what does it offer Halo to be a GAAS. For the campaign, it offers nothing but subtractions - so that can straight fuck off. For the multiplayer, though, it offers the potential for lots of interesting developments... but if those developments fracture the community with paid-content drops and egregious micro transactions, then it's only a detriment. Horrible cosmetics that ruin the aesthetics of the experience - armour straight out of Warframe a good Halo makes not - would turn me off in a big way.
I'll have to see their proposed implementation before I could judge anything. On paper, though... I'm pessimistic.