Important to remember a few things here: First, all of the St. Laurent derivative classes were built to DDE standard at first, except NIP and ANN, which were built to DDH standard right from the start, and then upgraded as either DDH or IRE. Were they supposed to be the last two Mackenzie? Possible. After all, the St. Laurent were a seven ships run, as were the Restigouche's, so a run of six would make sense. Remember also that in the end, not just the Mackenzie's were not upgraded: ST. CROIX, COLUMBIA and CHAUDIERE were also never converted into either DDH or IRE's.
The General Purpose Frigate (GPF) mentioned in the Crow's Nest article above were not the IRO's, but a class that was to be built between the ANN and the first IRO. It got canned in the freeze of equipment purchase in the lead up to unification. The IRO's were also to be an 8 ship run originally, then cut to four to be able to make updates to the design for the next four. Thus, they, with the GPF (total 16 hulls), were meant to replace the St. Laurent and the old Crusader/Prestonian frigates.
Further note here: Between the new builds and the large refits and upgrade, you may notice that the RCN of the 50's to 70's was in a continuous build mode. That was because it had a central office for naval construction, made of mixed civilians and naval officers (arcs and engs) based in Montreal at the Canadian Vickers yard. It not only centralized all plans, but developed new ones and was responsible for distributing construction to various yards. This was keeping no less than 6 yards in operation that could expand and contribute warships in case of war: Canadian Vickers (Montreal), Marine Industries (Sorel), Davie Shipbuilding (Quebec), Halifax Shipyard, Victoria Machine Depot (later known as Yarrows) and Burrard Drydocks (Vancouver). Since the RCN made its own plans for ships and their replacement, had its own HQ in Ottawa to push them directly into the orbit of the government of the day to get them approved and we were close enough to WWII for people to remember how critical warships were, it worked fine.
All this came to a screeching halt with unification and naval requirements being "just another demand within NDHQ - mostly led by the Army types - from one of the various command, and it'll go in the pile."
The RCN barely managed to save the four IRO's from the grind.