Exactly they would switch to other coin which has more volatile price and traders would like that and its not only a small portion because big investors do also like on bitcoin because of moving prices and if bitcoin would stable its price then popularity would somehow decrease.
Try to read how forex traders are getting profit out of the stable price of fiat currencies and you will have some enlightment.
If bitcoin price is stable, the trades is still profitable. Try to learn new things mate.
But how they are making a profit if the price is very stable i think if the price is increase 1% a month they are making profit?
Honestly i still not try forex trading since they are asking for high investment or deposit..