There's a big difference between a closed source proprietary technology like VB and GWT. GWT can't really die as long as people continue to use it, anymore than Perl, Python, Ruby, or PHP, or even libraries like jQuery, can die without some company funding them. On the other hand, if MS drops support for something, and there's no open source, that's it, the product is dead.
If Google ever abandons GWT (which they are not), the fact that it is an open source project means that anyone can fork it and continue to improve it as long as it is useful to them.
This has nothing to do with being screwed for profits. Google doesn't make money off of GWT and never did. It was an open source gift to the community that people were free to use and improve. GWT Designer, for example, use to be a commercial product from Instantiations. Google acquired this company for a large sum of money, turned around and gave the product away for free, with source code. In fact, it donated a big chunk of Instantiation's former products to the Eclipse Foundation.
I really don't understand how building your own framework from scratch changes the equation. If Google, for example, were to completely abandon GWT, it's still open source, so you can take it and modify it as you wish. Writing your own compiler and framework from scratch will take more work, rather than just forking GWT and starting off with a mature base.
We obviously don't encourage forks, and would rather merge improvements back into GWT proper, but the threat of a fork is always there if we don't serve the community well enough.