Technically you would still hold the key as a user, stored somewhere safe. You just wouldn't use it daily, as all your signing would be done by these providers in the cloud. So if they disappear you can get your key from storage and use it to create more shards. You could also distribute shards to multiple providers, like 5, and make the signature be possible with just 3 out of the 5, and so on.

I don't know about the incentives, it would be almost free for these providers to support a new user, so I'm not sure payments will be needed. I mean, Blockstream has been providing co-signing services for Green wallet users for years at zero cost, for example.