I wish there were something like nos2x but for mobile devices to make this many-app style future more manageable. Maybe it wouldn't have a great UX though if it requires switching app views just to sign. Every app that requires an nsec import increases risk.
Discussion
Amber exists: https://github.com/greenart7c3/amber
Fedi enables signing of mobile web apps through their mods feature
There are apps like #Amber for Android that implement NIP-55 which is basically NIP-07/nos2x for mobile devices - is this what you had in mind? I recently built a CapacitorJS plugin for NIP-55 and you can make the experience seamless for end users, they only have to authorize once - https://github.com/chebizarro/nostr-signer-capacitor-plugin
NostrKeyring.com is almost fully written and open source on my Github. Anyone reading this who wants a much better signer, please contribute.