What is stopping it's gradual integration?
You just said it. It's a gradual process. Rome wasn't built in a day. Everyone here in this forum knows that Bitcoin is a legal tender already but I guess you're referring to it in a more global acceptance.
As long as those who use it believe and accept it as money, It's money. I don't think it will get to that point where everyone will be forced to recognize Bitcoin as legal tender. No matter how a maximalist we want to be, we should also make room for others to have a choice not to like or want to use it. Despite how influential the United Kingdom is, the British Pounds isn't a global currency. But that doesn't stop those who value and use it from using it.
As you said, as long as there's someone using this currency and believe its purpose, this market will continue.
Worry if no one is talking anymore about this market, and seemed to be forgotten. But for now, it is like the adoption is further increasing as more and more stakeholders are getting involved in this market. It may not reach to the point where all jurisdictions are utilizing it as legal tender, but I would say, more population will be into this market one way or another.
Utilizing btc as legal tender may be easier in small countries where the government can easily get a hold of their citizens as well as their financial transactions, and with seemingly weak local currency. But for larger countries and have a strong local currency, they won't think of changing their position when it comes to their currency.