We know of folk in the ecosystem who are running older versions of zcashd. We don’t know if they are using `disablesafemode`. We don’t know what their upgrade schedule looks like. Thus I think sending out an alert a few days beforehand could result in the alert itself being a disruptive event with unforeseen consequences.
Sending an alert a few blocks before Overwinter activates (10-30 mins) would suffice and help prevent users from mistakenly trying to transact on the soon-to-be forked-off legacy chain. Also the ecosystem already expects some level of disruption when Overwinter activates.
From a communications perspective, this is also less confusing to users than trying to remember two different dates.
I would prefer not to send out any alerts and would like to see the alert system removed from Zcashd as soon as possible, for the purposes of security and decentralization, but given this is the first network upgrade and there are still a large number of non-Overwinter nodes, this may be a necessary action for the time being.