The line is when it becomes greed. In most cases 1 property is not greed while a lot of cases the 50 properties would be greed although not always.Where do you draw that line?
One person buying one property as a buy to let as an investment isn't any different, mechanically, to one person buying 50 buy to let properties as an investment.
There are 2.6 million landlords in the UK, most of which own one btl property. That has a pretty big impact on a total uk housing stock of 24 million.
Many of those 2.6million landlords will be covering people like transient workers, students, people who don't want the responsibility and costs of being a home owner or people who don't plan on staying in the area and want a temporary location for x amount of time. Providing a service that is needed and is helpful is not immoral and greedy like you have been saying. Take Streets tenet a transient worker at the nearby hospital who are just looking for somewhere to live temporarily. How is providing reasonable rent to that transient worker who doesnt want or need to own a propertie in that location classed as greedy and immoral?