Since I don't work for VMware I wouldn't know what the future of Onyx might be.
Needless to say that Onyx will not work, in it's current form, with the Web Client.
And since the vSphere Client is apparently destined to disappear, they will have at least to rewrite Onyx or come up with something new.
True, the SDK forum is not very active.
But that doesn't necessarily mean nobody is using the SDK.
Programmers probably use other media to communicate than administrators 