I don't think this was/is being handled by GM well at all. Air intake ducts in many cases last a lot longer than 3-4 years on most cars out there (some you never replace), so GM should have just covered it from the get go. I got the letter too and as I noted in another post, when the head gasket went bad on my 2012 1.8L auto LT sedan, I told GM that they were eating it (they said it would be $150 to replace it, I told them no, I'll get GM's zone rep involved). Sure enough they did......
The Sonic is in a very crowded small car market and in my opinion it has done little to stand out and at least from the customer service perspective, GM's "old school ways" shine through. Fortunately, like many here, I do my own work on cars and know when I think the dealer isn't being 100% forthcoming.....