For those with established .mobi's, is there anything really stopping the adaptation of these for desktop display?

Creating the css along with the handheld detect, there are two main things I can think of:

a) the issue of character count. Possibly that rankings could be hard to get with such economical text counts. If a conditional statement/css places added text, would this nix Google indexing despite the separate platforms?

b) the long term viability of the extension. Will it be around in 5-10 years?

I don't think the extension is a deterrent in itself for getting traffic for "typical" results.