There is no such guarantee, and if Bitcoin gets overtaken by new technology - something which appears to be already happening (given how much Bitcoin's market share dropped in a mere few months!), they will very likely end up holding the bag.
So what is your suggestion? To sell all bitcoins we have? That sounds like advice from someone who want to buy some cheap bitcoins, nothing more.
None: everyone should make their own decisions.
Your response is one of the things I've been worried about most in this board since 2013: the lack of (self-)criticism to challenge certain viewpoints, and the tendency to immediatly paint those that utter it as people spreading FUD, or that have some other interest that involves seeing Bitcoin die. It has enhanced the cult-like behavior here, as valid criticasters at some point get tired from talking against a brick wall.
Hodlers are nothing but a victim of the climate it creates.
What then would happen if
all bitcoin owners sold at the peak? I guess it would create a huge dip. And next, what would happen if all previous btc owners bought at the huge dip? I guess another peak, and we all could sell again and buy again and sell again and buy, all of us all at the same time, perpetually.
How would that turn out?
Doesn't work that way. Long term investors create scarcity, which increases value over time, which results in small price increases over time. Then, when the price is high enough, traders should sell, which increases short term supply, price goes down, at which point traders should buy, short term supply down, price up - up and down and up and down forever. Very exciting indeed.
All the while, us laughable hodlers, buy a little more, month after month, year after year, secure in Satoshi's vision. Don't get me wrong, traders play a very important role, like teaching novice traders valuable lessons for instance.