Hi Oleg 
I work for Discourse, so first off thanks for choosing us and kudos on a great design 
As an employee of an “open source company”, please allow me to add some positives of going open source for which King the company is the primary beneficiary. In short, I think going open source adds a large multiplier to the existing reasoning behind going free, as stated in " Why are you releasing Defold free?" on your front page.
Huge boost to your hiring pipeline
We believe great tech attracts great talent. All in all, we believe this ultimately leads to better games – not only for us at King, but for all game developers.
Open sourcing your code is the best thing you can ever do to your hiring pipeline. Especially for the hiring of new engineers, but also for just about any other position in your company. We’ve been steadily growing since our inception 5 years ago, now at 35+ people and a 100% retention rate for anyone who’s been with the company past their trial period of a couple months. The vast majority of our hiring has been done directly through our own community, i.e. meta.discourse.org. It’s really a beautifully organic thing to see someone go from
- occasional helper of troubleshooters to;
- tinkerer;
- intermittently sponsored contributions;
- paid trial;
- part of the team! (and on your first day everyone kinda goes “huh? I thought you were already working here…, anyway happy to hear we’ll be seeing you around even more!”)
The value proposition of your product grows in lockstep with your community
It may seem out of the ordinary, that a commercial game company releases its core technology for free, but here’s how we see it: the more people who use Defold, the better the engine will be.
By releasing Defold to the community, everyone can help making Defold better, by creating tutorials, by finding bugs, improving the documentation, and much more. And since King uses Defold internally, every day – the better the engine gets, the happier our internal developers will be.
You already have a wonderfully vibrant community here on forum.defold.com (again, major kudos is due 
) and going open source would put the momentum of your community into hyper drive. A more active community means more of all the good stuff:
- community-driven product development
- continuously improving organic knowledgebase
- vastly increased hiring pool
- viability for an ecosystem, should you ever want to go down that path
- a strong community mitigates the fear of a company suddenly going down and taking all their support channels with it.
Open source improves your product
It should and is not about free labor, it’s about polish. The vast majority of open source contributions are one-offs, be it code or many other things. But these contributions add up to a degree of polish that you’d never have otherwise, because there’s never enough hours in the day for those “nice to have” tweaks that never make it to the top of your TODO list.
You don’t even have to start off being open source
Going from closed to open, you have much more leeway in your choice of license. None of the open source opinionistas are gonna give you a hard time about going from closed to more open (okay there’s always someone, but it could be a lot worse), so you might even consider starting off with a shared source license like License Zero’s Prosperity License. You could continue to sell the license to use Defold for commercial use for 0$, but you’d have the right to change that in the future if you so choose, just like now.
Bonus: It’ll make extensions a lot easier and more powerful
You’re already going down this path with your extensions. And if I’m not mistaken, it looks like going open source with the engine itself would make it easier to roll out extensions. It’ll allow you to put some stuff out in a pretty rough form and find out how many of your users want this enough that they’ll go the extra mile to figure it out and tell you how to make it better. The open source mentality is a huge boon to product development because it encourages you to push out new code as soon as it works but before you’ve fully figured out where to go on with it.