Quorum? There is no voting for a soft fork. Miners either upgrade their client or do not. In a soft fork, all old nodes can see the new blocks, can still have transactions sent to addresses that they recognize, and can spend their existing coins.
Actually soft fork often means this:
1. A super majority of miners start to ENFORCE the change.
2. Not all full nodes enforce the change, but allow it.
New minting would clash with any of the 6000 full nodes that did not upgrade.
SegWit would not from my understanding because of "hacks".
Failure to attain super majority from miners would mean the change could not be enforced unless everyone upgraded (unlikely if you can't even convince 5 mining pools to begin with).