I have four inside/outside cats. Two of the cats have always been that way and when two others came along, there was no way to let the first two keep being inside/outside cats while keeping the other two as inside cats only. It felt cruel to keep all four as inside cats only when the other two had been allowed out all their lives. At times when I've had to keep them in after an operation or for any reason, they will cry and scream at the door and actually make their paws bleed, trying to get out. So they're allowed out in the morning and come inside at 5pm. I have learnt the hard way though, I lost a cat, Peppa Boy, when he was hit by a car and after I moved house, into a very quiet street, my lovely Honey Boy was also hit by a car and died
So, I believe, if you can do it, keep a cat inside, as long as you provide lots of toys( homemade ones are great like toilet rolls, balled up paper, boxes etc) and lots of attention. No matter how quiet you think your street is, one car is enough to cause heartbreak.