Now do your remaining cats a favour and make them house cats.
This IMO opinion is cruel (unless you own mansion with your own fields and stuff).
As someone once said, "Sure a house cat lives longer. It's a furball that can only sit in the window looking out at a world it'll never enjoy, but yeah it lives longer". Point being, it is better to die young but enjoy your life than to live to an old age but never have any fun.
All living things will die eventually, at least the OP's cat had a great life outdoors exploring and being in it's natural environment. What's the point in being a cat that lives 20 years if the whole of that time your are imprisoned in one house and never experience the outside world?
When humans start doing that we call them Agoraphobic and get a doctor to speak to them so why is it acceptable to do it with a cat?
Cats are not there for your amusement or comfort, they're there to be a cat first and companion second. Keeping a cat locked away all day because there's a very tiny chance that it could get knocked over is nothing short of selfishness IMO. You are putting your fear about losing the cat above the cat's right to do what it does.
Having said all that, I'll never understand why people who live on busy main roads keep cats, that's just asking for it.