It's an economic argument/fear that the pie is fixed and that value can't be "grown". Anything that isn't bitcoin is stealing potential value inputs to the demand and price of bitcoin.
I don't think they consider the multiplicative effect that things like Steem and Ethereum do for expanding the net.
I agree. Also I would add not just expanding the net, but expanding bitcoin and bitcoin adoption. As of real recently it seems as if Steemit has done more for crypto interest/adoption than anything else yet. Kind of sneaky, sort of carrot on a stick. Like "hey kid, you want some $$??.... Well you will have to learn how to change this crypto asset that you earned from your post into dollars on an exchange". I love it!