All of the reasons that justified upping the number of days back in 2013 remain true today: http://blog.newsblur.com/post/61451693579/upping-unread-stories-to-30-days-for-premium
But the cost of premium has gone up, and upping the number of days would be a nice perk to go along with it.
Whether or not the number of days is increased, it would also be nice to receive some sort of notification that unread stories will soon disappear (maybe 5 days beforehand)? I can envision an unobtrusive, dismissable bar at the top or bottom of the screen indicating how many stories will expire in the next few days. Or maybe there would be an option to get the warning as an email?