Nice idea! However, I still struggle to see the advantage over "!g".
The workflow is exactly the same: Search for stuff in DDG, and if you suspect that Google might have returned a better result, prepend "!g " to your query (which might be even faster than clicking on the bookmarklet, depending on how fast you type).
To be clear: while this button directs people away from DDG to Google, it actually allows people to use DDG as their first (main) search engine, and lets them switch to Google if necessary without much inconvenience. The better the DDG search results, the less views they lose to Google.