For me and i don't if everyone is like my opinion i think that if there is no other coins then the exchanges will have less profit and the bitcoin price will also decrease and many coins has a purpose altcoins are depend on bitcoin but not the other way trading is very important part of bitcoin it gives more coins volume and i always agree that bitcoins need an alt coin
if there were no other cryptocurrency than bitcoin, then there were no other exchanges (altcoin exchanges) to make any profit (not less profit) because bitcoin/fiat exchanges usually don't have altcoins.
and their altcoins volume is just a side profit for them not the main big thing.