Interesting blog post....I will say that email has it's place in the marketing channel of a business, but I hope it's automated. If your not using services to automate your email and or drip campaigns it would be hard to justify open and click rates in email now days.
I will say this if I have talked with the customer in the past and already got my message across and email follow up is a much better tool as follow up. We currently send out 7 touches via direct mail, 12 emails monthly at random intervals, text if opted in, voiceblast, Facebook Ads, Social Media post on all large platforms, and a few websites that rant well in SEO.
When taking into account how many touches are required to make a sale you really need to be on multiple platforms. If you customer is of the younger demographic it might not make sense to send mail, but the older demographic still values this, especially if hand written font is used.