Just remove USD from the picture - what would be the price for HERO than?

HERO references USD so it's somewhat still required for price reference.

What would show all those price feeds from our lovely witnesses when (and if) USD will gone?

We could certainly switch from USD as the reference asset to another reference asset, in the case that USD is no longer a functioning FIAT cryptocurrency the world will be completely different and the price of BTS may be astronomical.

Can the feed be zeroed, and where will be those 5% per year than?

If USD fails as a fiat currency, we could potentially switch to an alternative reference asset after resolving the global settlement via the BSIP-018 recovery mechanisms.

The scenario where USD has failed as a FIAT currency has failed is unlikely to occur, if it does occur then people will possibly flee to crypto away from fiat.