A bubble is not about how high is the price of something (tulip bulbs, diamonds, dot-com stocks...), it is when the price is not justified.
I think those two things are inseparable--a tulip bulb was definitely worth some price back in the 1600s, but the increase in price had gotten so out of control that people eventually realized how irrational the market for tulips had gotten. And if you look at price charts of things that have been in a bubble, you'll see the same thing: exponential growth for a period of time followed by an enormous crash and a long period of stagnant prices.
I don't see bitcoin as being in a bubble right now. There were "mini bubbles" in 2014 and 2017, but the recovery periods have been extremely short, and I wouldn't expect that in a true bubble. Look at the price of gold and silver leading up to 2011, when they crashed. It took 10 years just for their prices to get back to where they are now, which is still below their peak in 2011. The NASDAQ composite took even more than that to recover from the crash of 2000--and the list goes on.
Bitcoin is not similar to a Ponzi scheme, it's an investment that does not promise anything.
I think most members here realize bitcoin isn't a Ponzi or a scam. That was my initial impression of it back when I didn't know what it was, but nowadays I think even the average investors sees it as just another asset class (cryptocurrency), especially given how much mainstream media coverage it gets.