I still think there are a ton of people who want something that is small and portable but still has gaming power. I could see a ton of college students getting laptops for college and then buying an external GPU so they can game when they get back to their dorm. Anyone who travels or moves around a lot could have a powerful gaming laptop when they want it, and a light and usable laptop when they want that, without the terrible battery life, huge size, and obscene price that comes with a full fledged gaming-only laptop.
A desktop is a big item to lug back and forth between a dorm/apartment and home. I tried this last year with my desktop; this year I skipped the hassle and left it behind.
Most people move their desktop as often as they move the rest of their furniture. For college students who dorm, there are small form factor cases that can still support full size GPUs and CPUs.
Many students need to take flights at least once a year if they study out of state/country or if they have some kind of internship program (my school made us alternative between school and work every 4 months, for instance). Even the smallest of the SFF cases (that support discrete GPUs) cannot be practically packed into a luggage for flights.
I know this because I own a SilverStone FTZ01 [1] and have tried to take it with me on a flight back home in May, but ended up giving up on it because it was simply too difficult to adequately protect in a luggage without going over weight limits.
A much smaller chassis with just a discrete GPU + power supply probably won't have this problem, and as a result, for my use case at least, it would be a much more practical option.
To get the more powerful graphics card in the desktop you must also purchase a case, power supply, processor, RAM, motherboard, SDD/HDD, keyboard, and monitor. I can't see this not outpacing the cost of just the external graphics card.
I'm treating a mouse as a must purchase for either.
Without having done the pricing, I'm betting that the laptop plus external graphics card might not be much more expensive than a low-end laptop + "good enough" gaming system. Going super high-end is going to be expensive either way and it may not matter as much since you have the coin to go all out just for gaming.
