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.

I also think like that, altcoins come in the field not to support bitcoin but they want to replace bitcoin, so if there will be no any altcoin then bitcoin price will be more higher as the demand of crypto users will be only bitcoin.

Yeah it will be hard to many users especially altcoin traders to earn big profit a month because i think that trading altcoins is one of the best way to get big profits. So if they remove that "altcoins" it will cause for a low progress for bitcoin because some of the users (i think) will not continue to use bitcoin because it will be harder to them to choose method that are equal for their profits in trading.

Just to answer you, it would have been better if there was no altcoins in the market.

Now we have around 650 off altcoins in the market who are eating up bitcoin investments. If there was no altcoins in the market, then bitcoin would have reached $2000 USD by now. Each person investing in altcoins, could have invested in bitcoin increasing its market cap.

as far as i concern,  with Altcoin Bitcoin became popular in a matter of short time during the altcoin rises up.
because its like a masses that has been promoting Bitcoins in any places in the world. and without it bitcoin will still stand firm.

Bitcoin was created before any of the altcoins that we know today. so any altcoin exists because of bitcoin and copying that idea. but also they are an alternative way of investment which only affect bitcoin amount that we all have.