The ones who say this thing that bitcoin is a bubble are not fully aware of the nature of this bubble, it is very much different from rest of the bubbles as it has this capability of rising high even after going down. All you need to have is some good knowledge regarding the things which can then help you in thinking of a way which can help you in making maximum profit from this bubble.
I do not think anyone believe its a bubble aside from some people who have no economics knowledge and just talk on a FUD sense because they lost on getting in on early. Those people who call bitcoin a bubble also are the same people who keep saying "I wish I invested back when it was $100" or so. I have been in bitcoin world for over 5 years now and ever since I started I have seen many people calling bitcoin bubble and every time they call it a bubble bitcoin rebounds back and breaks the ATH levels. So not worry and just carry on.
At the beginning of the Internet, many people said that it was a bubble. Isn't this Internet a bad thing?
The term bubble is being used with the sense of "something is growing big without proper reason for its growth". If something is growing along with proper back-up, that is the actual growth which is just the opposite of bubble. Internet has proved its worth over time hence it is not a bubble.