If you aren't paying anything for the product and there doesn't seem to be a business model for it, why would you assume it won't go away at some point?
Again, it's about reputation. Most users don't want to have to try to examine the motives of the people behind every product they use to decide if it's likely to stick around. Most users have neither the savvy nor the desire to analyze business models and see if they seem sustainable. They want to say "Google - that's a solid company, so it should be safe to depend on this."
And if Google keeps invalidating that assumption, people will stop making it. Instead, they'll say "Google is famous for discontinuing their products, so I'll hold off on using this until I know it's really, really popular."
And that could make launching new services very difficult for Google in the future.
