besides why should we compare market cap of these compare to the market cap of Bitcoin? How it does make any sense? I'm really curious - I just can't get there. Maybe I'm not so big financial brain but I simply can't get there.
Because the market cap (times a velocity factor which is normally of the order of single digit) for a monetary asset determines its scope. It wouldn't mean a lot to compare to the market cap of, say, Apple or so. But it does mean a lot when compared to monetary market caps.
The upper limit of bitcoin's market cap is of course the world's M2 market cap. There's simply not more money in the world (savings deposits, cash, bank accounts, ....). Bitcoin can at most replace all monetary assets. It can't replace production capital for instance. It can't replace an airplane. So that upper market cap brings bitcoin in the few million dollar of today range.
It would mean that there isn't any fiat money in the world any more. It would mean that all fiat money in the world, on bank accounts, cash, on savings accounts has been replaced by bitcoin, and doesn't exist any more. No more dollars, Yuan, Yen, Rubbles, Euros.... all of them replaced by bitcoin and no altcoin. THEN, and ONLY THEN, a coin will be what is a few million dollars right now.
As for the gold, cash and savings I think it's valid - coz Bitcoin is close enough to gold, cash and/or savings accounts.
Indeed. But you should realize that bitcoin can only take on their market cap if the asset under consideration has ITSELF lost its value. So Bitcoin will replace gold's market cap when gold will have become worthless and you can exchange a kg of gold for a pizza.