These things are going to become more popular over time, not less. Desktop sales in general are plummeting and will continue to do so. GPU cases unify the (growing, healthy) laptop market and growing NUC market.
Few examples how these can be used:
1. You can share 1 expensive GPU between multiple family laptops and a NUC at 1 desk rather than outfit 4 people with Geforce 1080s.
2. It's also great for say a college kid, you need a laptop for school and maybe you want to play games in your dorm but don't want some massively expensive huge gaming laptop? You get a Thunderbolt GPU case.
3. Someone who travels for work, needs a laptop but again just wants the top-end hardware at home rather than continuously buying the latest $2000 gaming laptop every year.
It just makes a lot of sense with no real negatives other than price, which will drop. It's still early days. Wait till Rosewill sells one that's just a case + TB3 interface, and you provide your own ATX power supply of choice.
That will be the tipping point for this market. People just need a while to get used to the idea, but GPU cases, NUCs and laptops are the future.