I don't think >90% of Gmail users would use that feature.

Sorry I had trouble parsing that. Do you mean that you think 90% would not use the feature, or do you think that the feature would be used by less than 90% (and is thus not worth being part of the default).

I think that over 90% would not use that feature.

buffer built an app on that feature for social media. surely there is a market for it except some folk may want to reverse it.

Compose mail aroun 9pm and send it at 1am to show they have been working all night :)