What's your own definition for "mainstream"? Bitcoin being used globally with no exceptions, risks, and limitations? Honestly, bitcoin is already a mainstream.

As long as the majority of bitcoin enthusiast will consider bitcoin as a "thing" that will make them rich someday, no way we can see bitcoin will act as a pure currency. Despite the big adoption (yes, I believed that was big?) in bitcoin taking place year by year, still, others take this as "early-stage" which is really not. The development of bitcoin is already can consider as a milestone. However, since people trade for it often and others hodling it, the supposed currency was primarily used as an investment. Big companies don't want the idea of a volatile currency that's why they are hesitant to accept it.

But you know, we can't blame people for that, even I.  Our options to use bitcoin as a currency is limited, at least at some places, that's why others ended up trading or take it as an investment instead.  Big companies or institutions must start accepting bitcoin for that volatile thing to minimize. We really need a whole world's cooperation for this to happen. So yes, because of that status, after all these years, bitcoin is still considered at the early stage of adoption.