It's probably worth being clear about that.
You might think of landlords as rich enough to buy property outright and milk it, but you would mostly be wrong.
If you were buying a house for the first time, to live in, and you were a sane person, you would take out a capital repayment mortgage, putting down a deposit but borrowing most of the money. Then you would pay off the cost of the house and the interest over the next 25-35 years.
Example 25 year capital repayment mortgage on £160k towards a £200k house: £632 a month.
If you're becoming a BTLer, you don't do that - at least, the majority don't. You take out an interest only mortgage over a shorter term.
Example 5 year interest-only mortgage on £160k towards a £200k house: £385 a month.
When your 25 year CR mortgage comes to an end, you owe £0. If your house price is £100k rather than £200k now, well whoops, you paid over the odds. But that's all.
When your 5 year IO mortgage comes to an end, you owe £160k, which you hope to be able to recoup if necessary by selling the house. But if your house price is £100k now, you're fucked.
The interest-only timebomb: How to get your mortgage debt cleared before it's too late