What is the problem when landlords are leaving the marked?

Its usually the counter argument said against any attempts of rent control laws or increased taxation.

But now.. if you

1) cap rents

2) introduce a law to say foreign people cant buy up local properties (already exsists in other countries ie Denmark)

3) tax empty properties at a very high rate (alread being done as well)

then what exactly are landlords going to do? Sell the place? great, the influx of new properties will crash prices so that suddenly young families can afford to buy again which frees up rental properties

Seems like a very easy equation to reduce rents with no negative consequences (besides reduced income for landlords)

What am I missing?