How much do the unhappy users pay to use GC? Not even a penny. It doesn't really matter how many users complain about Bing maps or how many decide to leave for alternative services. Nothing times a billion is still nothing. On the other hand, Garmin must be getting something from Microsoft. Switching to Bing maps was a business decision which would never be reversed. Unless of course the decision turns out to be affecting the business negatively.
I would go to major internet shopping sites such as amazon.com and write product reviews for Garmin products to let the potential buyers know that Garmin Connect no longer uses Google maps but Microsoft Bing maps and how you like Bing maps so that they can make informed purchase decisions.