North's spine is very good, it's just that Waite is 35 and Thompson 32, so if you take them out they come back to the field.
We're in a similar boat. Our spine is our strength but last year Naitanui missed the whole year and Kennedy was out for 6 games and we were middling at best.
We're middling with them and we would be middling without them though. Our midfield is the main difference between our bottom 4 of 2017 and mid-range finish of 2018.