> One way out could be to tax the value gain at the time of sale

This is how it works for sales in Sweden, via the so called "reavinstskatt", or profit realisation tax. Funnily enough, it is mostly applied to individuals in practice. To avoid the taxation, companies package properties in subsidiaries, which can then be sold and traded like any other stock. The tax is then postponed until the de jure sale of the property, which never occurs. Individuals are not eligible because the loophole is closed if the legal owner also occupies the property they own.

This sounds like a plan to make rented properties a more liquid asset and make rent cheaper. Seems like a sound idea to me.