I have a modest collection of about 100 U.S. plate blocks from 1945 to 1973. I only collected plate blocks of four, and only of engraved designs that I considered to be the most attractive, and that exhibited superior engraving workmanship.
ikeyPikey, You were right on with your comment about the plate block market crashing with hoards of sheets coming out of storage and onto the under-face postage market, flooding the market with plate blocks when the rest of the stamps on the sheet were separated for postage.
The multiple plate numbers per sheet printings were obviously intended to bilk collectors out of their money for cheaply-printed stamps that would presumably never be postally used. Below is a 6-plate-number example (US Scott #1444) necessitating a 12-stamp plate block that is the exact opposite of the attractiveness/workmanship that I sought when building my plate block collection.
Without factoring in inflation, post office revenue has been impacted in the recent decades with people like me using their stamp collection for postage instead of buying new issues. I also collected sheets to a lesser extent than plate blocks, but I may be dead before I postally use all the sheets I bought.