Dungeons & Discourse's "I watched the D&D movie. It scared me.":
DM Lair chat with Roll for Combat, with the reason for 6th edition at the timestamp:
https://youtu.be/OKQOhnHOg6I?t=3296
DnD Shorts, "Every Insider Leak I've Been Given on Wizards of the Coast":
0:00 Intro
0:22 Did the movie flop?
1:04 Hasbro's aggressive profit goals
3:23 Why this means aggressive monetization of D&D
9:04 Hasbro's last great hope
11:34 Implications for 6th edition
13:12 The actual reason for a 6th edition
15:21 Final thoughts