"Remember, in the 60s they expected these systems to be replaced within ten years. Hence the millennium bug..."
Eurm, not really or not only. 6 digit dates were used to save space on storage media. An IBM 2311 disk had a capacity of 7.5MB. A 2400 tape stored, depending on blocking factor and recording density, 113-170MB.
So shaving two bytes off each date stored for an application could save quite a lot of (scarce) space. Also Y2K was 30-35 years down the road. I don't remember many people thinking that far ahead. The assumption too, as you pointed out, was those current at the time applications would be replaced some time in the future.
I remember doing a gig in the early 1970's for a large insurance company whose master file resided on 100+ tapes (tape because the data center's raised floor couldn't handle the weight of the necessary disk units if the file had been stored on disk). So cutting down on storage requirements was a must.