> If not ICO, why would I pay the Ethereum network to deploy my contract, develop an app, promote it, and then convince my users that they should pay the network to use my app, too (assuming that I hold 0.0000 of Ether and don't care about the rise of its price)?

If you want to enforce the same properties that the Ethereum network can, but you don't want to use anything associated with the Ethereum network, then you need to write something similar. And it turns out this takes work.

So you could either:

(1) write your own separate distributed cryptocurrency system, which is not exactly a "crush some Red Bull and hack it in a weekend" project; or

(2) use Ethereum, which already exists.

As you don't have to do it yourself but you still benefit from it, some people like (2) instead of (1).