That's a good point - they don't need to write that all themselves but the maintenance burden is certainly significant.

That's partially the reason of using ancient non-GPLv3 software that's often no longer maintained upstream, meaning even more maintenance.

Personally, I think this another bad decision on the same level with keeping some their own code closed - it just means more work for them and hinders community participation. And as with the closed components (Silica for example) I still don't see any benefits from this for Jolla.

Sure, but do all of their backers understand that ? There is a significant amount of hype around the whole thing, so I'm afraid backers with less experience with mobile platforms and/or distribution development might be setting their expectations a bit too high for what will be possible to run on the devices on release and how well it will work.