OP I don’t think you’re being unreasonable to expect the rent to be paid at some point as essentially you are being kind and subsidising their housing at present and you’re not obliged to do this ad infinitum or indeed at all.
You are subbing them, that’s why you feel like that. They’re paying less than the property they’re living in is worth. Are they paying back the discount you’ve given them?
I am worried about a downward pressure on rents, so wouldn't it be a good idea to kick out the current ones and get some white collar workers with secure jobs on a years contract?
Some peoples attitudes to renters is so messed up. As a PP said (commenting on the last quote above), some LLs don't actually seem to see their tenants as people: they're just irritating non-entities who happen to be paying you two or three times the monthly cost of your mortgage each month, mainly because you were in a position to get a deposit together to buy a house and they haven't yet been able to.
The PP was right: it is a business, but it's not the same as something like retail or providing a leisure service. People happily pay you far more for a single chocolate bar than you paid for it because they don't want to buy 1,000 of them. They will happily hand over £4 for a coffee from your shop/stall because they aren't at home to make one or can't be bothered to anyway. If they prefer not to do either of these, people can easily choose to forego them.
Housing is much more complicated. Everybody needs one home and the vast majority of renters only do so because they don't get the financial break to be able to start buying their own house and paying a fraction of the monthly rent towards a mortgage.
I'm not against people becoming LLs at all (I was an accidental one myself some years ago - incidentally, my excellent tenants' circumstances worsened through no fault of their own and I dropped the rent by a big percentage). I just think that many feel fully entitiled to have it all their way at the expense of much poorer people than them paying a premium.
As a LL, your mortgage has nothing whatsoever to do with your tenants. Of course, the balance of finances has to work for you longer term, otherwise you will have to reconsider your business and maybe eventually sell up if the figures no longer add up; but you have no automatic entitlement to have your monthly mortgage payment covered in full by your tenant's rent, let alone two or three times over. As has been said, their rent is helping you to buy a property. In theory, you could have the same tenants paying you for 20-25 years and pretty much buying you a free house whilst they have nothing to show for it all in the end.
Why is it such anathema to even consider that, should the tenants' monthly rent leave a temporary shortfall in your monthly mortgage payment for the property, which you then have to make up from your own funds, you are still in a very fortunate position in your longer term goal of owning a house outright at relatively minimal cost to yourself?