If Google deindexed the spam, that effort might actually be worth it for a bike shop to do an honest comparison. But so long as you’re just going to get buried by SEO spam, why bother?
While that would improve things I still think it's rather unlikely that a bike shop would be willing to give anything but a great rating for the bikes the are selling. And if they are their suppliers might not like it. There's still a dependency.
True, bike shops tend to hock one manufacturer. I still think reducing the SEO ‘reviews’ would create space for honest reviews though.