I would love for something like this to become mainstream, mostly from a privacy PoV. But, I think there are just too many different use cases, pitfalls, user problems and limitations. Something like a plug and play box could work for the very small intersection of "technically inclined and understands how to port forward and how to fix minor problems" and "not interested in tinkering with self hosting". As someone else already mentioned, a big part for people self hosting seems to be the tinkering aspect. For me tinkering and privacy are the main motivators.

On the potential problem side you have far too much friction compared to cloud services. It starts with getting a public domain and IP for sharing data with other people. Then you need to setup port forwarding, need a somewhat stable network and internet connection, run into problems with upload bandwidth being usually far less than download. Once you have the connection side taken care of, what about user onboarding, password recovery ("just mount the partition and overwrite the password" will not work), backups? How do you make clear the distinction between my self hosted Spotify and someone else's? Why can they not see my music? Why is there a difference in the first place?

I know all/most of this can be solved in some way, but it is hard. There is also never going to be a big (think billion dollar) company involved with creating such a solution as there is no monetization model. You can not go for a subscription (imo), because then you just pay someone to host something at your home using your power and bandwidth. You could say you only pay for support and maybe a relay to workaround NAT, but how would the support look like? Full access via an admin account? Privacy just got downgraded. What happens if the company hosting the relay goes under? Enjoy your brick (local would still work, but the user experience would be far worse).

Apart from all these software and user experience problems, we still need the hardware. It needs to be reasonably cheap (US and EU market maybe 200-300USD), but it needs to be reliable (it will live in the worst possible place, like a hot, dusty cupboard) and support some more advanced use cases. Of course >90% will only use it for some light file storage, music streaming and voice control, but 10% will make heavy use of transcoding, invite others to their services, store TBs of media, run their entire home automation on it. How do these 10% know they are in the 10% and need to buy different hardware? Why can the box not do automatic quality adjustment like Netflix does? Do you have a hardware migration path?

And what about cross compatibility? In this thread alone you got multiple different systems. This just multiplies user support issues or will lead to vendor lock in, just a different kind.

Lastly, you need marketing to bring this to the mainstream. And you need a big selling point to make people migrate from "free" cloud services to something they need to pay upfront.

I hate to be this negative about this topic, but I just do not see this being viable outside a relatively small, interested community. Please, correct me here, I would love to see this become real and mainstream!