If you've actually followed him on Twitter for more than a day, you'd know he's long gone from webOS and into iOS/Android pastures now and doing just fine in that world.
The only thing he did was finally formalize it, and bugging him with endless messages isn't going to further your case when he's told people outright on Twitter that webOS development is something with no clear direction for success at this point. He has no interest in coming back short of a miracle involving Open webOS becoming commercially competitive, and that also has no evidence of happening--so it's not even worth considering.
For every developer that formalizes their dropping of webOS, there are several others that just do it silently, stop their updates, and concentrate their efforts on other platforms. All you have to do is follow and look. I mean, honestly, do you really expect Flashcards to be updated ever again? Or any one of hundreds (or perhaps thousands) of other independently developed apps that once enjoyed success?
Many webOS developers even took their knowledge of Enyo and ran for other operating systems as soon as it became feasible to do so when the Enyo Project began supporting other platforms. And they haven't looked back.
[I]Most importantly, stop annoying the developers that move on.
You're far more likely to drive them further away with your zealous desperation than convincing them to come back. Developers go where the excitement, progress, and money are, and largely aren't interested in charity cases for orphaned mobile operating systems where they can't make money*
. Open webOS and especially legacy webOS just aren't worth most developers investing any time in anymore and if you fault them for it, it's not their problem. If webOS ever becomes a platform worth considering again in the future, they'll be the first ones to come back.
[I](* Hint: The only developers making more money on webOS than other platforms are 1) those that only develop for webOS, and 2) those that don't understand the audience of other platforms at all, and 3) those that don't both to use any of the user interface conventions or user experience constructs of other platforms when porting their apps. Users either won't trust them or understand them when most of their other apps tend to have at least some UI/UX consistency.)