Thanks for the feedback!
What would that incentive be?
The incentive is that your data is being shared. As you share data from others, they share yours. Reciprocal sharing.
How would you handle spam if the content can be updated?
It's not like an email service, think of it like a web page. If you go to your favourite-webpage.com and suddenly there is something you don't like on it, you can just opt out by not going there any more.
What about mobile users, who have a data plan limit, low space and battery to worry about (so no daemon 24/7)?
This is not made for very low bandwith systems, or bandwith expensive systems. This said, I have a 8€ monthly mobile plan with IIRC 210GB monthly data (then it gets throttled), so if you do not yet have access to low costs like this, eventually you will.
If you want to share a 1MB website (which is quite big IMO) and share it 10 fold, you'd need roughly a whopping 11MB of storage. Again, everyone does not have lots of free storage, but the trend seems to point towards that and most phones today have that storage capacity.
This is a first version, but nothing prevents a low power usage version to be made, quite easily too.
On mobile, my concerns would be more of the possibility to have a port forward, and an IP address that doesn't change too often (like many many times a day).