[sundaeofshock]

Putting the API behind Twitter blue might not have worked. The paid badge has become a mark of derision amongst a portion of the user base.

[ascagnel_]

It would go a long way towards making such a service acceptable ("I'm only paying for Blue for Tweetbot!"). The other option would be to charge developers for per-user access to the API, which developers could build into subscription payments (eg: Twitter could have charged Tapbots $2/user/month for access, so Tweetbot would have to charge their users $31+/year on iOS).