For security reasons there should be no public rpc access to witness node. Witness node should not run anything other than it is required to generate blocks.
Full API nodes are not supposed to generate blocks, they are broadcasting transactions that witnesses include in the blockchain.

Using Steem platform is incentive good enough. Usually when you need full API node, then you already have some some project around it. If that's just a startup and usage is low, then it's fine to use public nodes (like my own). If such business grows enough that it needs own node(s) then it should be viable at that point to hire sysadmins and buy appropriate infrastructure.