It's a very good piece. What I will add, from the political side of things, is that making things better for private developers is an easier sell if you're investing in council housing as well. So it's ultimately not a choice between one or the other – it's both or none.
I mean, this is essentially another example of trying to get the private sector to do social policy and it going badly wrong. If you want more housing at social rents, get the state to build and finance it. Don't tax private developers into oblivion and choke off economic potential in the process.