There is nothing official in bitcoin world because there is no centralization to issue anything "officially".
As for the
main usage of bitcoin, it is still as a currency and the nature of a currency is also demanding store of value and the limited supply plus the increasing price makes it so that some people use it for an investment.
I thought BTC was going to replace
You thought wrong. Bitcoin was not created to replace X or Y. It was created to fill a gap in our world for a medium of exchange that is not and can not be controlled by any centralized authority.
I guess thats the point of stable coins,
The point of (centralized altcoins commonly known as) stablecoins is to let exchanges and users who don't want to go through the hassle of the banking system for their many deposits and money transfers between exchanges to have an option to do that. Not to mention reduce the risk of their bank accounts being closed. With stablecoins they reduce the number of times they deposit and withdraw fiat and reduce their risks that way.
Will it only be used for investing/speculation as long as its price is this unstable?
Each time bitcoin price goes up the number of purchases taking place using bitcoin increases to the point that the ATH of price is also the ATH of bitcoin usage as payment so your argument is moot.