Alright. It seems there's no real disagreement about how the opcode behaves. Perhaps a time limit would be appropriate to stop people creating outputs locked for 100 years .... is bitcoin even likely to exist in 100 years? The entire history of computing is not even that old, seems hard to imagine that it'd be good for anything beyond wasting space in the database. But this is a minor point.

So I guess it's time to start the deployment discussion.

Bitcoin is a consensus system. It works best when everyone is following exactly the same rules at the same time. A soft fork works against this principle by allowing nodes to think they're following the majority ruleset, even if they aren't, effectively downgrading them to something a bit like SPV security without them realising.

A hard fork has multiple desirable properties. Most importantly, it means a node can detect it's no longer in the consensus because it'll find its own chain height has diverged significantly from its peers. Core already has code that knows how to detect this condition and log errors about it as well as running the alertnotify script i.e. emailing the admin. Ideally it would also stop serving work so miners shut down or fail over, but this is easily added to the CheckForkWarningConditions() function.

In other words, this gives the cleanest failure we can give, such that any procedures a node operator has put in place to alert them of divergence will be triggered.  Any code which is waiting for confirmations will wait forever at this point, thus minimising the risk of loss.

Additionally, forcing old peers to fall behind means SPV clients will pick the right chain, and not end up downloading transactions or blocks that are about to be doomed at the next re-org. They can easily choose to ignore transactions relayed by peers that are too far behind and thus not end up accepting transactions that are no longer valid according to the majority (a scenario which can cause monetary loss).

I don't think hard forks should be scary. Mechanisms are in place to warn people and they can be scheduled with plenty of time in advance. The main stated justification for a soft fork is backwards compatibility, but in a system like Bitcoin you really don't want to be running behind the consensus and it's hard to imagine any node operator deliberately choosing to stay on the wrong side of the fork. It's not like other software where people can choose to skip an upgrade and things still work just like before.