A couple of articles about sustainability:

https://medium.com/cryptozoa/how-is-this-sustainable-a-closer-look-at-the-taxes-of-the-drip-network-50adb2c8192d

https://medium.com/cryptozoa/is-drip-sustainable-65c5c4b2f79e

This was written by @Andrea over in Drip Rocket Science. It addresses some of what you're asking about:
"Drip sustainability is about game theory and non inflationary tax system. The vault is the heart of drip and pays everyone who decides to claim and you can keep an eye on it looking at his analytics. Is not much about equations per se but much more in a proper equilibrium to find in all the choices everyone choose. The vault is fed continuously by all the taxes applied to all transaction and there is mathematically always more inbound tx that outbound ones. The vault cannot be easily drained because of how the liquidity works on the internal Dex and its relation to bnb and its price. The vault is also fed by the taxes applied to the drip busd farm which was significant in l1 and will be huge in l2, and by all other projects in the ecosystem that will be release in future. The vault can, when and if necessary, mint new drip to keep the system go on at the cost of start being inflationary. There are a lot of details in all these but game theory and tax system mostly sum it up. I'm a cs/math/programmer for profession so it's fun to dive into details of things."