I don't see the relevance of right to buy. As you say, it may have reduced the supply of social housing (a good thing - ridding the taxpayer of a liability) but it didn't reduce the overall housing stock. It's also worth adding that it was an extremely popular policy at the time - and I know quite a few nominally working-class people who benefited greatly over time from having a valuable asset to their name which they would otherwise have never had a chance of owning.
As for all these unused plots, I'd like to know where they are. All I read about in my local paper is loads of towns and villages campaigning against large numbers of new houses being built in their location. And looking at how local infrastructure is already at saturation point, it's easy to see why people feel this way. London has already had a wave of overspill last century which the south-east was able to take. This time it can't.
Why is getting rid of social housing such a good thing? No, it hasn't reduced the overall housing stock, but what it has done is made the stock that does exist unaffordable for the people for whom it was originally intended - ie ordinary working people.
These days we see social housing as "housing of last resort" for those who are on benefits, or are on such low incomes as to be able to afford to pay for housing at "market rates". The whole point of what we now see as social housing was to address the issue of people being unable to afford decent housing at market rates.
I don't fundamentally have an issue with right-to-buy. If you've lived in a property for a long period of time, paid your rent on time and regard the property as your own home, that's fine. I don't have an issue with you being offered a discount on that home when you do buy. What I do have an issue with is when the people that have been fortunate enough to be able to afford to buy their homes outright - easy to do when you had enough savings - then sell them on for a massive mark-up.
We're the left with the situation where the remaining council stock is reserved for those most in need (ie homeless), and those ordinary working people who would have normally been considered for a home are left out in the cold, at the mercy of "market rates".