Also, I think the important point is: let's just not say if your 'grace period' is too short, what if you see 5% of nodes hanging out there and have not upgraded? 15%, 25%? What if they still haven't upgraded after the fork because, well their admins could just go on vacation or something? What would happen to all the SPV clients connecting to them? Are you going to back down in this hypothetical situation?
If they won't upgrade, the new clients will not even sync with the old relays, because blocks will be seen as invalid by the clients.
If we are talking about SPV clients, which don't see any blocks, then their money will also not be lost. They will regain the control of their money once all of them (relay/processor + client) upgrade.
The money will not be lost in (almost ?) any case.Also, generally when running business it is your goddamn job to make sure that everything is ok. If you don't know that, you should not even start a business.
The grace period is actually much much longer, because the whole situation will be widely known at least few months before that happens.
So companies actually have months to change, not weeks.Well, I think that covers it.