Yet a huge amount of people base the value of bitcoin only of the value compared to Fiat money.
They're speculators. It's inevitable that they appear in such a free asset.
Speculation
I agree with this point, so let's move on.
Unstable value
As long as the value is not even near stable it's not a alternative to Fiat money. Why would you buy anything with it as you believe it could be worth double the money next month? Same for the opposite.
Why would you sell anything for fiat money knowing that it'll be worth half as much in twenty years?
The scam of Trading platforms
I've been following trades on for example Bitfinex, there are people who know how to influence the price for sure. Some big players (you call them whales) who laught at all the noobs while they are just pumping the price to a new high. It's far from transparent.
This is a short term problem. It may result in crashes, but decentralised exchanges are in the making and regulation for the centralised exchanges is getting tighter. In fact, that's why Bitfinex no longer deals with US customers.
I think you're right that BTC's value will decrease, but your claims appear to suggest that it will be a permanent decrease, which is something that I disagree with. They're primarily short-term problems (within the space of a year or two).