Governments can't control it, yet El Salvador decided to make Bitcoin legal tender. Ironic, isn't it? Imagine how things would be if institutional investment firms and governments amass a large portion of the circulating supply. BTC will be highly unequal and less-decentralized. To be able to buy/sell BTC, you'd be forced to use a centralized exchange or service as the supply lies in the hands of such entities. So making Bitcoin legal tender would have little or no relevance at all.
Governments can control many things including national policies on many industries but they can not force citizens to do whatever they want. Citizens have freedom and they always want to have freedom so more or less, citizens in any nation will try to chase freedom and dislike unreasonable mandatory enforcements from government. Government can think a policy is good and serves citizens but if citizens don't see it helpful, they will have many other choices and ways to adapt to and stay away from that mandatory policy.
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.
Bitcoin maximalists want Bitcoin to dominate the world, and there is only Bitcoin in blockchain and cryptocurrency market but it's unrealistic to have that complete domination. Bitcoin can not kill governments, central banks, commercial banks and altcoins and the reality is Bitcoin must co exist with other things around the world.
Some countries consider it as legal tender, some consider it as legal, the others consider it as allegal or illegal. People can consider it as money, digital money, investment asset, store of value, whatever they see good but all these things are very nationally and individually, not mandatory.