There is no mathematical or logical truth in your post. Here are the answers to your questions.
a) why is BTC quantity limited with 21 millions? What limits it?
Majority consensus of the users limits it. If a minority tries to change the limit, the minority is exiled.
b) what is the point of decentralised money, if its so volatile, that rate would drop 75% in 6 months (or grows by the same)? this is called a hyperinflation.
Decentralized money removes the control over its use by any third party. Volatility can be a problem now, but it won't always be a problem (hopefully).
c) why is blockchain technology better in 99,99% of transactions, than regular private/public key technology?
The blockchain removes the need for trust in a third party to manage a common pool of value. Bitcoin uses public key cryptography, but public key cryptography isn't sufficient all by itself.
d) do You really understand, what is private/public key crypto and blockchain in its core, or they just sound cool?
Many people don't understand it, but understanding it is not necessary in order to use Bitcoin.
e) if BTC had at more or less decent chance to boom again, why dont all the richest investors in world buy it?
Many are skeptical, and many oppose it because their wealth and well-being are dependent on the survival of legacy currency systems. Either way, unanimous belief in the success of Bitcoin is not required for its success.
Maybe it wasnt planned so, but BTC is the most genious scam in the history.
In order for it to be a scam, someone must be the scammer. Who is the scammer?
1) The blockchain part or "mining" in case of BTC is a special kind of scam. For getting a coin You must invest into electricity and computing power. In return You get VERY little chance to be awarded with a coin. This chance is so small, it is mathematically ridiculous, but in practise. ZERO! Nada, ничтo, nichts... What kind of decent money is that, that everyone can apply by spending real money but only few of many millions are actually given by basis of pure luck?
It may be small, but it is not zero, even in practice. Many people earn money from mining, so it can't be zero.
2) The coin part of BTC is a monetary fraud. Although people make transactions voluntarely, they are cheated with the very core concept of BTC. While blockchaining may have some usage as a concept as well as cryptocurrency, it does still not mean that BTC is legit.
It is not monetary fraud. Where is the fraud? People are not cheated by the concept. How can they be cheated by a concept? BTC is legit. You have provided no information at all to back up your claim.