If you search for it,
you'll realize that there isn't a standard definition for the “global adoption”. Bitcoiners tend to use it in their sentences as a way to describe Bitcoin's adoption by everyone on every part of the planet,
hence “global”.
You are overthinking everything these days.
Reaching "global adoption" which I prefer calling "mass adoption" simply means at least some people from any part of the world have adopted bitcoin. Not just in one country or a handful of countries like the developed countries while the rest remain unaware of bitcoin.
Sad truth, but Bitcoin can't have a global adoption same like computers or internet had decades ago and I state this from an economic aspect.
Both have "global adoption" just at different levels depending on the location. For example you can find computers and internet in Africa the same way you can find it in America.
The more users this thing has, the more the speculation.
Actually the more users means more stability because if we get more speculators who do day trading the market gets more packed and the order books aren't so thin so that one or two whales could dump into to crash the price that easily. The effects of both manipulation and panic sells decrease.
Early adopters/investors get the chance to be included in the peak of the pyramid and earn more long-term.
You don't "earn" anything by being early. You just increase your
purchasing power as the deflationary currency you own gains more value.
~ that the majority sees it as an investment.
You can't claim to know what the majority sees and your reference can't be the vocal people on the internet who keep posting about price and bitcoin as an investment. For all you know the "majority" who see bitcoin as a currency don't like screaming about it on the internet.
(In the system below, 100 fresh bitcoins are brought into circulation every day.)
Let's assume that once upon a time, on an island, there were 100 people with 100 mushrooms in total and each one of them had 10 bitcoins, willing to use it as a currency. This means that each mushroom costs 10 BTC since there are 1000 BTC in total. But, hey, the next day, there are 1000 newcomers bringing 1000 mushrooms and they are also willing to use Bitcoin as a currency. But, unfortunately, there are only 1100 bitcoins in circulation which means that each mushroom now costs 1.1 BTC. Same thing would happen if that little community gained 10,000 new mushroom-farmers the next day. Every mushroom would cost 0.12 BTC.
Well in real world (after reaching mass adoption) the equilibrium doesn't just break that easily, specially not by 1000%. The new adopters coming in or new coins being created won't be enough to disrupt it.