bitcoin having a fixed supply doesn't means it will continue to go up forever, all stocks also have a limited supply of outstanding shares and stocks don't go up forever.
That's correct. But if all millionaires on this planet want to get one single Bitcoin, it's not possible, as their are more millionaires than coins.
Some stocks are inflationary, like Tesla that keeps issuing more equity. If a company does a buy back with free cash flow then it it is deflationary but if it borrows money to do the buy back then it is just swapping equity for debt.
Same goes for Bitcoin it was just 21 million coins then they issued Bitcoin cash and Bitcoin gold tripling the supply and they may do it again and again . The fixed supply argument is a fallacy . Not to mention that other alt coins are in a fixed supply as well . It's all artificial not genuine .
Who is "they" ? And don't you need a very different Future-contract to short an Alt-coin as well. Ahh.... I see you think you shorted them all at once with one CBOE future? {if not, then you admitted you understood that Bcash is really something else and there is only one Bitcoin with 21 mill limit}.
"Not to mention that other alt coins are in a fixed supply as well" The supply of Ethereum and I think Ripple are not fixed and can be increased.