And personally, I think keeping them inside or on a lead is cruel.
You could probably argue is both ways, or take the view that no cat, kept inside or kicked out in the yard, is having the proper wild animal experience. It’s possible we’re taking an anthropomorphic view on what makes a cat happy.
My view is that cats are territorial animals, and feel happy and secure when they are occupying, controlling and patrolling that territory. If that territory is a house and a couple of gardens, fine, but it can also be just a house.
It’s quite possible that an ‘outdoor’ cat will be more stressed and unhappy, particularly if there are other rival cats looking to occupy his/her territory. A quick google suggests that hundreds of thousands of cats get run over each year in the UK, too, which is worth taking into consideration alongside wildlife massacres.
As long as a house cat gets opportunities to hunt and exercise through play indoors, I don’t see why they can’t have a good life.
Not sure about leads, though. You’re letting them out but not letting them out, so they’ll be desperate to go out and patrol properly.