Indeed, there isn't a precise metrics to gauge how widely Bitcoin is adopted. Some may choose to consider the quantity of daily transactions or the number of Bitcoin wallets that are currently in use. However, these gauges are limited to certain factors.

However, despite its rather low adoption rate, Bitcoin is nevertheless having a significant effect on the financial system. For instance, the collapse of FTX has effects in both the traditional finance and cryptocurrency sectors. Therefore, even if just a small portion of people use Bitcoin, its impact is far bigger than that statistic would imply.

 Well, I think On the one hand, Bitcoin has been around for more than a decade now, and it's had a number of major price fluctuations, crashes, and comebacks. So it's certainly not a new technology. However, until it becomes  commonly used for transactions, there is still a long way to go. As of right now, the majority of its users are investors; regular people use it less frequently for tasks like paying bills or buying groceries. Therefore, I believe it to be in the middle of the "early" and "mature" areas.

It also appears that ignorance of the underlying technology contributed to some of the mistakes committed in the past. For instance, the Terra/Luna scenario resulted from a crash caused by an algorithmic stablecoin design error. Also, FTX's inability to accurately account for customer deposits was a contributing factor in their bankruptcy. I don't really think there are many fundamental flaws with Bitcoin itself, I believe it's more about how it's being implemented, which I believe could be readjusted as the day goes by.