Usually when mortgage interest + etc. > rent, it indicates that the property is overvalued and the value (over the long term) is more likely to fall than rise. Every other buyer - or almost every other buyer, except the ones that need to launder money - is making the same economic calculation, and if the cost of money to buy the property is more expensive than the rent you can earn from the property, they're not going to buy the property.

Ah but my equities-heavy roboadvisor portfolio has gone up way more than the landlord's apartment...

is it so? Is that still the case when your landlord leverages 20/80?