I don't think anyone should be too surprised that there is a rough plan of Marvel films through 2021. When I first read it a few days ago I wasn't. Feige said it best, what we planned for 2015 back in 2006 has happened. Long term planning is par for the course for Feige/Marvel Studios.
Like the editorial comment inserted by the writer of that piece I too think Defenders seems like a likely path after an Avengers trilogy. Strange, Hulk, Namor(beat Aquaman to screen also) and they may have Silver Surfer back then.