We are still in the early days. Once adoption comes, upgrades should be mandatory only if a security bug is found.
On-Chain tech is getting 10y old and proven now. If the protocol gets not messed up again (noSW!) than BCH wins world adoption!
"mandatory," good luck with that.. mandatory implies an authority.
Nothing can be mandatory in this sphere, but things can be obvious.
With mandatory I mean releasing an upgrade without backwards compatibility in case of bug. Miners and nodes with economical activity are incentivized to upgrade anyways.
