For storage platforms like Dropbox or Onedrive, it can be as simple as ctrl+c'ing the data out of there every now and then
For Spotify, you can do the GDPR export and check that you can open it somehow. Even if it's not super readable, you can figure that out (with a tech friend or LLM perhaps) if you turn out to ever need it
For Signal, I've got no idea. The format is hard to work with for hardcore techies. They really really don't want you, ahem, attackers to get that data in the clear. (Two-sided coin). Make sure to turn on the backups, write down the passphrase, and take them off your phone every now and then, and hope there's a restore method when you need it
The only reason I can imagine why nobody implements this, is that they like the status quo where it's hard to switch to their service, but equally hard to switch away. Once this becomes commonplace (e.g. because vendor A makes users aware it's a thing by implementing it for migrating from vendor B), vendor B will "retaliate" and A will have to truly be better than vendor B to retain said users
That is all to say, this might yet totally be a viable and legally unblockable business avenue for a third party to initiate. I'm just not sure there's enough demand to warrant (read: pay for) the effort involved. Maybe if you can have an LLM read the api docs of the service where you want to do the import and autogenerate a mostly-working version?