Any bunch of people coming up with a radical idea can call themselves a think tank. (Are we one?)
LCY is a very viable and valuble business caterring for mainly business passengers in what is a very efficient use of 500,000 square metres (or 50 ha (0.19 square miles, or 123 acres) that generates £750m from the economy which actually goes to the low paid rather than bankers.
Like all businesses, if it's not making a profit then it'll cease to be viable and it'll close. Then we can build houses on it, or more likely apartments for the well -heeled. (cetainly won't be 'affordable housing')