Mercedes would never let us make their car into a bad Decepticon. Car company's really frown on that - I can't say what the game company and comics do that's their own thing. They don't have the same constraints movies do. It's Carly's car.I have never even heard this new Soundwave theory until now. A trailer for the DOTM video game indicated that Soundwave's alt mode might be a SUV while another one suggested the security truck might be it but really we have zero information on the character. Considering the Decepticon's tend to be mostly CGI creations both in robot and vehicle mode I doubt we will find out for sure until the toy leak or the book comes out.
As for Bay's explanation, that is pretty much right on the money. The Binaltech/Alternator line experienced this difficulty first hand. The line was complex Transformers with alt modes based on existing cars on the market at the time. To make the toys, Hasbro and Takara had to get a license from the various car manufacturers and most would not agree to allow the car to be a Decepticon resulting in a very Autobot heavy line. About the only ones that didn't seem to mind were Japan automotive companies like Honda and Mazda. Everyone else balked at the idea for fear that consumers would associate the villain with their brand. It is a nonsense belief that has never been proven but if you are in the marketing department it is much easier to say no than risk the yes.
If Bay really needed a Decepticon vehicle, a Japan auto company probably would have signed on but since he already has the Autobot fleet and a lot of very expensive sports cars to choose one from as long as not the bad guys, well can't fault him for taking advantage. Besides part of the fun of the Decepticons is by mostly avoiding cars, they provided a more broad choice of alt modes to enjoy.