Until those studio's running proprietary engines show their versions of Nanite, then UE5 is a giant step ahead at the moment. That's not to say the cuting edge studio's like Naughty Dog won't be able to give fantastic experiences tied to more traditional or contemporary development practises, but the two UE5 demo's have shown that this is clearly 'the big step forward' for this generation and if you don't have similar tech, then you need something like it. At least in my opinion. Decoupling the poly budget from the background environmentals is the start of something great and hopefully it will apply to non static meshes at some point in the near future. Nanite can be applied to almost 90% of most scenes apparently.
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In terms of constraints, most studio's are still going to be limited by the vessel and the art teams. BD-XL is the maximum size of your game on disk to ship which gives you 100GB. That's not much if you want everything to have a 4k texture and even worse if you want 8k textures in the future. It's a static contraint for all team and platforms. There's nothing stopping these packs being optional of course and downloadable but you'd be more likely to see a medley of different textures in that case (bundles of 2k, some 4k and a very few 8k).
Using Nanite will give you almost endless budget for all the static meshes but for the dynamic stuff it will need to be creative (static mesh overlays for things like armour plates or robots, providing you didn't need deformation), or still heavily authored.
Around the 11min mark they discuss Nanite meshes - so they can respond to physics, but the plates won't deform when hit etc.
Things like foliage etc. are also exempt, so that's still going to rely on traditional development and art as well. I think ultimately it's going to come down to who can blend the two techs together and avoid 'uncanny' valley syndrome compared to those studio's using their own optimised consistent and proven workflows. But I would be surprised if they are not looking at Nanite or already have something in their engines either that can do the same job or they are working on to emulate the same.