In my opinion it’s unlikely to be a single reason for everyone, and the two subquestions may be red herrings: it’s unclear why broad adoption would require adoption by GAFAM and others, and the comparison with other apps presumes that what is possible with Solid is always possible with other apps.
As with any technological adoption pathway we expect only early adopters to begin with, and we expect laggards later, and in between a lot has to happen to shift the economics and make the value proposition more and more attractive.
Finally, current UX is not a good predictor of future UX. We’re still at the stage of non-versioned breaking changes (!) and from what I hear there are still many breaking changes to come, which we expect as the technology matures (hopefully within a clearly versioned process).
There are obviously other lists of advantages but here’s a few points to get the imagination going:
Solid is a strong proposition for people currently on the edges of the self-hosting movement who want control over their web services with less effort. Currently people either have to install many backends or they buy home servers or SaaS offerings with a limited software availability. Solid makes it so you install one piece of software on the backend and you have access to interoperability with all Solid apps.
Solid only requires frontend development, so the barrier to entry is much lower. You don’t have to commit to running a server at all. Both bigger and smaller cloud players already offer incompatible options in this space and the server-less model is increasingly popular.
At the moment platforms like IFTTT have to wrap a whole lot of different APIs each with their own authentication. With Solid I’d expect abstractions over private data to become increasingly user friendly, so it becomes trivial to create automation rules that take action not just on your data but also on others’.
The point with privacy is not whether people are concerned by it, but rather what becomes possible when people feel in control over how their data is used.
At the moment it is inconceivable to, e.g. set up an automation for, e.g. a doctor to provide health information directly to insurance companies and for the insurance company to then provide information to your gym, with all drawing on your wearable health data, and therefore for everyone involved in your health to actually have access to all the information needed for proper coordination of activities and incentives (including prices). If the user has control of this process, it becomes possible, and this would be all thanks to allowing simple consent workflows to be built on the underlying Solid UX of allowing clients access to your pod.
Web monetization is obviously an enabler here. I still find it incredible that the value GAFAM gets from individual users is often measured in $ not cents, and that people can make their living from cents from many other people. Decisions about control over information are likely to be informed by value of information, and the more common that becomes, the more opportunities there will be. Today software like the Brave browser pay the user for ads. When we get to the point when users have the perception that they can get free money just for consenting to provide their data, that’s going to be a game-changer for adoption too…
There’s still a long road ahead, but there’s plenty of opportunity in the future.