Him continuing to develop the game for another 7-8 years is fine. The only thing that worries me is if he suddenly loses his motivation or gets a writer's block and has to take time off (although I don't think he's taken time off ever since he completed Acting Lessons) and suddenly this masterpiece ends up incomplete.
7-8 years is unsustainable. The culture and people change too much over that period of time to maintain an interest in anything that isn't widely promoted and hyped. Eventually people will move on to the next best thing.
The other worry is the GRRM issue. The episodes are getting bigger and bigger, and eventually DPC is going to have to wrap everything up. If episodes continue to grow at the rate they are, he will have too many loose strings to tie up neatly in the end. This is a classic writer's 2nd work mistake - you've written one work, and now you want to take your writing to the next level, but you bite off more than you can chew. He's walking right into the trap.
DPC would be much better off with a tightly written story that is the focus of these episodes. Things like DnG and months developing a phone aren't going to maintain the fanbase, nor will this story continue to be engaging with the same characters over the course of 7-8 years. He'll either have to add more characters and conflicts (causing the writer's knot to get more loose) or tighten up the story and move to a closer scope of narrative.