Why would you NOT have a mailing list?
I've personally seen Adsense Marketers go from $93,000 a month to zero almost overnight because some search engine moves the goalposts
(Some of you reading this might even have been hit by that if you've been around long enough)
I've seen SEO experts watch their money sites vanish from rankings in days when Google does an update, never to return
And I've seen Facebook pull ad accounts that have been making thousands with very little explanation
But if you have traffic on tap in the form of a responsive mailing list - an audience you can contact easily and for free - then whatever happens you're pretty much going to be OK
That's why you need to build a responsive list