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.
I agree with you, but I do think the original goal of bitcoin was to create a mainstream payments system.
About the block size, it's not only that. As transactions/userbase grows, the operation cost (I mean, more miners will join as bitcoin becomes more and more popular) grows exponentially. Proof of work just doesn't scale well.
Maybe I'm wrong, and satoshi did create this as an inital proof of concept.