Don't assume I'm talking about hours, or just the shows I have quoted.
In any event - 1975 just had three channels, not breakfast time TV (merely stuff for schools), and a switch off @ midnight with the exception of the O.U on Beeb2. So no bland fillers back then - just a test card.
Do the math - the hours you have assumed are way off for the era.
Add in other comedies and drama's and you will find that it is in no way sparse.
If you do the math for last year on current schedules - I bet you will find it's sparse. And until we actually get someone in the know to do this - we cannot definitively say one way or tother.
Plus you start adding in OTB (I know I know!) Bless this house, The Doctor in the ...etc, Man about the house, plus the drama's like Upstairs Downstairs, The Sweeney, Rumpole. Angels, space 1999 , Doctor Who, etc etc - your theory don't really stack up I'm afraid in the context of the era.