The Infinity is a fine setup, it gives you a finer degree of control of comfort in a house.
Payback period? There isn't one.
Installing an Infinity control and zoning system as part of an HVAC replacement can make a lot of sense when the AC system and furnace are both Infinity compatible.
A two stage heating thermostat capable of controlling your humidifier will give you much of the comfort control of the Infinity and will not require the replacement of your zoning system.
I am for doing good to the poor, but...I think the best way of doing good to the poor, is not making them easy in poverty, but leading or driving them out of it. I observed...that the more public provisions were made for the poor, the less they provided for themselves, and of course became poorer. And, on the contrary, the less was done for them, the more they did for themselves, and became richer.
― Benjamin Franklin