I understand the reason for picking GPL for the editor. The, rather large, downside I see to this is that effectively all plugins are now GPL, correct?

I run a small side business sell packages for Sublime Text and considered the concept of something similar for Light Table. While you certainly can sell GPL software, you can not prevent redistribution, so you effectively get into the business of providing support. I can't foresee being able to run a small support business without catering only to the enterprise. And even then "small" would be a relative term.