The Alts have made tremendous gains vs Fiat. When BTC / Fiat is at its peak, don't the Alt / Fiat have to collapse? People will be selling Alts to buy BTC. Right?
A peak has nothing to do with it, generally when BTC/fiat is increasing (bitcoin value going up) then people sell their alts to buy bitcoin and the alt/fiat prices fall. But right now despite being close to an ATH bitcoin price has been stable for around a week and that's what's allowed altcoins to have this great run they're enjoying.