It doesn't matter how large the market cap is. Bitcoin defined itself as a currency - currently, it is a very limitet one. Very far from being mainstream for a currency. For an asset? Sure it is mainstream. But bitcoin defined itselt (at least, satoshi did) as a currency - not an asset.
If you talk about Bitcoin as a mean of payment, then I agree with you, it's not a mainstream and will never be because it's not designed for that. Bitcoin was created to simply get rid of 3rd parties and create a decentralized, P2P transaction in ecommerce.
To be honest, I think that Bitcoin was most likely a test project than a final solution to the problem that Satoshi wanted to solve. There are 7 billion people in the world, 5.5 billion of them have access to internet and Bitcoin's block size is 1 MB. It's impossible for Bitcoin to become a mainstream currency because 1 MB block size is the limit. For it to become mainstream, we need to change it's protocol, so still, I wouldn't say that it will never become a mainstream because it can, we only need to modify things.