From what I understand, they are porting some established software like Qt and GCC, and presumably if there is enough interest and volunteers, more applications

From what I understand, they are porting some established software like Qt and GCC, and presumably if there is enough interest and volunteers, more applications. However they also aim to write a core set of apps specially for Serenity from scratch, so it seems like the best of both worlds to me: a set of native, integrated programs designed to showcase the OS's specialities, with ports of popular FOSS programs, and since Serenity does aim to be "Unix-like", it shouldn't be too hard. The incumbents in the field are absolutely overwhelming in their existing functionality, which makes it an extremely steep slope to climb for an upstart, unless they aim to fulfil a specialised niche. Linux struggled for years with lack of drivers. That's always the one of the chief roadblocks. If the OS can drive a broad range of hardware, then automatically users and ports of software will happen, but if it can only recognise a few generic devices then it will remain hobbyist. But thanks to the complexity of modern hardware, the task of even adapting Linux's drivers would be an enormous undertaking.