I do understand the view that you 'don't fix it unless it's broken'
BUT...Who want to find out while towing at speed on the interstate??
OLD tires equal RISK!! What risk you are willing to take will determine your course of action.
Not long ago, I replaced ALL (six - count 'em all 6!!) tires due to age - NOT cheap!!) The tread was only 1/4 down but the age was a risk for me in our travels.
I'm sure the tire shop sold them to a trucker who will get many more mile from them - but then that's his risk!!
Listen to the DW and your little voice inside that wants to minimize risks on the road!!