It is more of trial and error in more numbers (e.g. more DApps on Tribes or SMT based). Due to sheer number of attempts, one or two DApps can be a big hit that will bring millions of users. Users may not be aware of the underlying Steem blockchain. DApp can provide necessary SP (e.g. 20 SP delegation) to new users to function in their DApps. In that case, that successful DApps will require millions of SP. The killer idea for that DApp is still to be seen: right now, @actifit, @steemhunt, @sprinterlands, @palnet, @steemleo have good possibility.