The problem is not the lack of extensions, but the fact you changed existing URLs. So now every existing URL looks like a dead link, which google does not like.

The problem is not the lack of extensions, but the fact you changed existing URLs. So now every existing URL looks like a dead link, which google does not like. Make sure you 301 any existing URLs to the new ones (should be fairly easy since it’s an obvious structure of extension to no extension)