If you really wanted to use the term upgrade, you might be able to use it
only for soft forks. Unless a chain is completely centralized, a hard fork is
always going to cause a chain split (read that again, always). That's why we call it a hard fork.
I think that chain-splits are not the most important things, instead the separation or un-consensus of core team's members or communities are.
Splits should be focused on splits inside the core team or communities. It is likely that we all focus on wrong point, simply focus on the effects, not causes of those effects.
No; none of that has anything to do with technical terminology. Stop spewing nonsense.
I have still thought that it is time to add some strict criteria to the term hard fork.
We already have that: Anything that causes a chain split is a hard fork. End of discussion.
There is no such strict criteria to the term yet. One would not the wrong to call the constantinople upgrade a fork
Constantinopole is a forced-hard-fork (as ETH is not decentralized), not an upgrade.