Compared to the competition, such as Corona SDK, King will have to work real hard to build a community and ecosystem around Defold.
In 2016, having a great technology alone is not enough to get traction in game dev, all other parts of the ecosystem matter a lot: language, plugins (own and third party), art assets, tutorials (written and video), books, mind share retaining efforts (podcasts, events and etc).
Unity got this quite right, others are working hard to catch up (Unreal, Corona in 2D space).
Being open source is a huge boost as well, in terms of flexibility of development as well as peace of mind for the third party devs that the owner of the engine won't switch to the dark side with the next leadership change or simply let the technology get obsolete by not releasing new versions any more.
Still, I welcome this, Defold is a cool engine - hope that King open sources this eventually.