> I would rather see mass-market external GPU enclosures become the norm once/if Thunderbolt 3 becomes mainstream.

Agreed, this would be more AWESOME, but there is a limitation on the width of the bus though. I recall reading this when it first was announced.

> dual-boot a Macbook Pro for work during the day, then come home and plug in an external GPU for gaming

Depending on what you play, with Unreal Engine 4, games can run native. Unreal Tournament runs fine on a OSX and on Linux.