If commercial usage of the language by companies big and small around the world along with funding of the ecosystem is not an indication of mainstream usage, then I guess we'll have to disagree on what constitutes mainstream.
Right, a few users don't make something mainstream. Big companies have thousands of developers - even non IT companies - that someone inside some big company uses a programming language doesn't make it an indicator for widespread (most people use something -> mainstream) usage.
Mainstream: large relative and absolute numbers of users.
