In order for bandwidth to provide significant financial value to the Steem blockchain, it has to be a significant cost to the app developers/users. The larger a cost that is, the more worthwhile it is for those developers to go through the hassle of their own blockchain. The lower it is, the less argument there is that it can bring in a significant amount of revenue/demand for the Steem token.
Also an SMT will be tradable for STEEM in the internal market. How and where will your token trade when it's a fork?
There are hundreds of exchanges where you can trade tokens of all sorts.
The server costs alone to setup a reliable fork of the Steem blockchain should be enough to seriously consider just buying/renting some Steempower.
Steempower you could also easily up or down scale depending on your needs.
It also cost money to get listed on an exchange.
It's just so much easier to use the internal market of the Steem blockchain that will be instantly available to you and to trade SMT's for STEEM there.
STEEM is being traded on so many exchanges now. Going this route make just more sense to me then trying to get listed yourself.
Perhaps for apps starting out, at which point they don't need that much bandwidth and thus don't bring in major revenue. Once they get big, those issues become trivial, and forking the Steem blockchain off with an existing app is not that complicated. At that point all the bandwidth on the new chain is available for their app primarily.