What's so bad about high-Z inputs? Designing around a floating input is the same as designing around an undefined variable. Bad design but you can't "hurt" the hardware.

An undefined variable is an abstract construct. You can model a floating input in as being analogous to an undefined variable and the model will fit reality most of the time, but the underlying reality is still governed by the complexities of physics, with potential for the model to break down.

Damage is probably more of a risk for an external I/O, but it's still a possibility for an internal I/O. Probably more of a certainty with a malicious programmer.

Who's going to be the first person to design an on-die switched capacitor voltage multiplier, using parasitic capacitance between gates as the energy storage, and use it drive an internal high impedance input to a damaging level?

The only tristate buffers in modern FPGAs are on the pins. There's no way to leave inputs floating internally.

Z logic or tri-state logic are not real in FPGAs, they are chains of AND OR logic

so basically you cannot burn FPGA by making a short-circuit

A floating pin on a CMOS device is absolutely real.