For a moment, I was thinking Defold ought to dual-license their engine under both the current non-OSS modified Apache and the GPL. That way, you'd have the option to either:
1. Commercialise software created using modified versions of Defold, without releasing the source of your modified version, as long as you don't commercialize your modified version of Defold itself.
2. Commercialise a modified version of Defold, but you must make the source available under the GPL. (Which would mean that source could be used by the upstream project as well.)
But while typing this up, I noticed the flaw in this plan—the parenthetical isn't true! Because the upstream project would be dual licensed, they couldn't use GPL licensed code.
Of course if they do, I hope they will say "either modified Apache or the GPL".
My company's lawyers made a big stink about us using jQuery plugins that said "and" instead of "or".
There’s no reason to assume that a paid fork would reduce the number of free Defold users; it can happen, but depends on what is built and offered, and sometimes paid forks are good for the ecosystem and increase the number of overall users.
If you need to add an extra API or something to the core to make your paid extension work, you can't charge for that, which I think is designed to incentivise "improve the extension API, contribute that back to the core project, then go wild on your commercial extension and see if you can get people to pay for it."
I have no clue whether this approach will turn out to work in the medium-to-long term, but it's a fascinating idea and seems at the very least like an experiment very much worth conducting.
For example a company might try to sell a version of engine which has been ported to a console which original engine doesn't support. Game porting companies are very common and if it's their main business then they will usually have inhouse libraries or modified engine versions which significantly simplify the porting process.
That's exactly what's happening with open source game engines like Godot. Their documentation lists almost a dozen companies providing porting service for godot games. That isn't necessarily a bad thing, but it's up to author of game engine whether they want to allow others to profit from their work in such way.
Seems like currently Defold supported platforms cover most of the popular consoles, it was probably not the case during early development of engine when license was chosen or in a few years when next generation of consoles come out. Someone might also be selling a better console support than what defold provides out of the box. Beside the consoles there is also stuff like integration with various PC stores like GOG,Epic and others. Its not necessarily a huge work, but plenty of smaller devs want to focus only on the gameplay aspects. So once a game is finished (and you are tired from development process), buying anything which significantly reduces porting/integration effort can be an easy choice.
One more example of major feature which can require tight engine integration and motivate buying a modified version of "free engine" is multiplayer support. Good multiplayer support can be quite tricky with some game genres being harder than others. There have been many attempts at providing magic multiplayer solutions which under the hood automatically synchronizes all game entities without developer thinking about it. Such approach isn't necessarily going to be as good playing experience as designing the game with multiplayer support in mind from day 0, carefully thinking how the game state is organized, what when and how is synchronized. But that requires planning ahead, technical expertise and suitable budget. Commercial multiplayer middleware for existing engines are also not uncommon.
Whether something like that is considered an addon or modified engine version depends on exact licensing terms and the exact implementation details how game engine and addon code is organized.
A slightly different example - game engine built on top of game engine is RPG maker. For a long time RPGMaker has been it's own game engine. But few years ago developers of RPGMaker made a version of RPGMaker which is built on top of Unity. Plenty of other genre specific engines (especially for fighting games) built on top of general purpose game engines. Again the line between modified engine, addon and game with builtin editor is tricky.
Edit: clarity
Why?
Tbh I'm a little more confused after reading this
