"What about replacing the double threshold with a hard 1 hour run? Seemed to work for lydiard... There are definitely combos that don't work but let's also not pretend we know the exact best levels of intensity/volume."
You really didn't digest the article by Bakken. There are many reasons to have the double threshold vs the many other things you could be doing to get max duration and yet cap intensity with a realistic time frame for recovery and repeatability. You would certainly not want to be doing a 'hard 1 hour run' for a 5k by bakkens analysis, especially considering the recovery needed, the speed implications, and the physiological costs vs the benefits
No one is pretending to "know" anything here -- this is the exploration of a system that has worked wildly for a certain country that through borrowing and trial and error has produced a general framework for sustainable aerobic growth with amazing results, most of them in the face of convention
Do they know the 'exact' anything? Of course not, that's not the point. The point is 'what's good enough, erring on the side of caution to promote year over year growth'