For end user software, it's not such a big deal to drop support.
But for a project like Basecamp, businesses are buying it and potentially investing a lot of time and money to integrate it into their processes. Companies buying that kind of software expect it to be around a while. It's a bit disingenuous to sell to them knowing it may not be supported after a short time.
Maybe I meant to say that departments WITHIN Google are run like startups not Google itself.
It is a corporate company. When millions of people rely on your products you need to be careful when introducing new ones and pulling old ones. I don't think you can run a business like a startup forever if you have millions/billions of users. You cans till maintain some aspects of that startup culture but there are other aspects you have to leave behind. You have to think of how anything you do is going to affect people's long term view of you. The Reader shut down may not have affected a majority of Google's customers but now some people are wary to adopt new Google services. If you pull that kind of thing often enough you damage your long term prospects.
My main point (and question) of my comment was in the first sentence.