Let me explain it from the point of view of my users. My Chrome extension Plus Minus is quite popular ( https://chrome.google.com/webstore/detail/pidkbnhjgdngcfcaik... ) and has some pretty hard-core users. Before the new change, my extension enabled users to mark posts as read (by collapsing them to one-line like most email UI), allowed you to control which circles showed up in your stream, and allowed numerous UI customizations (widen the stream, fade out the distracting right sidebar etc.)
Last night at 1am I updated my extension because a minor G+ change broke it. This morning I got flooded by notifications because the entire G+ UI changed and my extension broke again. G+ users installed Plus Minus because they truly felt G+ lacked some key features they found necessary. Plus Minus allowed hiding users from specific circles before G+ made a feature to adjust per-circle weight on the main stream. Try using the old or new G+ on a wide LCD and notice the large empty spaces where content should be.
Users do not care if I'm not using the official API or directly making changes to the DOM. They only care that my extension works well enough to improve their G+ experience. The philosophical question here is whether a website owner should encourage, discourage, or be ambivalent towards 3rd parties that improve a user's experience via client-side plugins. Everyone supports data-sharing via REST APIs, why not client-UI? Having a decent TOS for the UI API (don't hide ads, don't auto-post etc.) should help keep it respectable.
Reddit has RES which drastically improves the user experience. I haven't seen any official support for it, though neither have I seen any official opposition. What I have seen are users who go ecstatic when they find out about RES. Why doesn't reddit say how they feel about RES? Why don't they support it via official UI-handles? Why can't G+, FB, and other large sites do the same? Greasemonkey is too greasy. Why not officially encourage UI-improvements?
To put it bluntly, I think I can significantly improve on the G+ interface for a typical power-user because my incentives are vastly different from Google's. I don't want the big Hangout feature all up in my face, regardless of how amazing it is. I would rather see better/more content and filter it well. My 10k+ users feel the same. Should Google support me in any way? I don't expect them to but it would be nice.
Just like Mohamed, I spent a considerable amount of my personal time on a free extension to improve Google's social platform on Google's web browser. I have nothing to gain from this except kind words from strangers. Yet I feel I have helped G+ in a small way. When my extension does not work, I don't feel like browsing G+. My users have said the same thing to me repeatedly, publicly. So while 10k user is statistically negligible for Google, there are many extensions out there and extension-loving power-users make a significant portion of G+ early-adopters and frequent posters. Maybe Google no longer has to worry about early-adopters and is going straight for the masses. Again, not my right to judge. I'm just saying I spent a lot of effort on making a piece of software loved by tens of thousands and instantly, it's all gone to waste. Mohamed has 100x my users so I can understand his frustration.
1. Your extension overpromises to users its abilities. You are responsible for that, not the users. You make no comment in your extension's description that it relies on undocumented, unsupported DOM manipulation.
2. The "fundamental question" has been answered many times over: website owners are (rightfully) ambivalent to such extensions. If you want one, great, but don't complain to us when it breaks. This is exactly what Mohamed is doing.
3. Even a million users is insignificant to G+, so it is fallacious to say that you or Mohamed "helped" G+. You optimized it for a certain subset of users.
It's decently obnoxious and entitled to think that Google should be beholden to people doing things without any contract, or even any offer, of support. This is pretty much the same rabble-rabble-rabble that appeared when Kevin Rose shut down Oink: "I used your service, you owe me." Except it's "I wrote an extension, you owe me." Google doesn't owe you anything, and it certainly doesn't owe unsupported extensions knowledge that would otherwise be under NDA.
Look at this from Google's perspective: they would like to improve the G+ interface for the users that don't want/need/know how to use your extension (i.e. the majority of their users). How else are they to do this - they cannot consult with extension developers on every change they make. Nothing would ever get done.
As your extension relies on scraping the DOM, you cannot expect to have stability, especially with Google's tenacity with their UI updates to Google+. You took the risk, and unfortunately, it involved more work than you would have liked.