As far as the 30fps in Destiny 2, I think the problem here is that Bungie is simply using an outdated engine based around last gen. Nobody wants to admit it, but it was an engine entirely built around Xbox 360/PS3, even came with several admitted limitations based off of that. Then we move to Destiny 2, which simply looks like Destiny 1 moved to Very High settings, better AA, textures, DoF, improved lighting, etc etc (a Destiny 1.5 expansion if you will)... besides that it looks like a lot of recycling going on and seems very doubtable that they've even had a chance to overhaul their engine.
I would say maybe they've been too busy building up content, but we have journalists talking about playing content they've shown for Destiny 2, but several years ago.. content that was even originally shown and then cut from Destiny 1, lol. Maybe they're just playing the tight budget/playing it safe.