On the other day, I think of the use of stable coins. I mean, when can we use Stable coins as an alternative fiat. You see when a bull run comes and everyone will start to sell their Bitcoins and some other coins into fiat the network will be congested.
So I think if we buy Stable coins with some stable volume, we will no longer need to worry about unable to sell our bitcoin into fiat.
Rather we only need to do this thing right? is this some acceptable alternative or no?
That is not bad idea - to invest in stable coin. But this stable coin should be trusted and have a good volume. Such good is like USDT or USDC. That is much trusted coins. Another good stable coins can arrive but need more time.
What the...? Invest into stable coin? Are you kidding? Do you think that you are really investing? It is something like to say, I am going to invest into american dollar.

You don't seem to understand that everything is an investment, if you are holding all your savings in your national currency then you are invested in that currency, and if for some reason it begins to lose value against the rest of the currencies then you are losing money.
It is the very same concept that allows you to trade bitcoin against the dollar and if you can sell it and buy it at the right time you can get more bitcoin in return, so I do not really understand why you are so surprised that something like that can be done and sometimes it is even a desirable thing to do.