There are over 800,000 second homes in the UK so the number is not insignificant & if even half of them became available it would have a significant effect on the housing crisis.I can understand the resentment that I think I detect in that post by Az. Younger people have a very tough time in paying the costs of putting a roof over their heads, but his prescription would not solve the problem and never could have. The numbers of people with multiple houses are small, but the shortage created by thirty years of mass migration and low levels of building, has created a massive shortage of homes. Getting a few people to sell off their second home would do nothing to reduce prices, and as you say would ensure there was no possibility of renting either.
The major issue is that we used to build over 200,000 council houses every year but 30-40 years of building no council houses while selling the remaining stock at knock down prices has led to a crisis in the rental sector.