Joe,
Would agree that all par 3 holes would be better than all par 5. Having a decent length approach shot is really the thrill of golf.....except for smashing a drive, which should save the par 4's.
Had a client once who proposed (but never funded and built) a concept called "second shot golf" where it was a 9 hole, par 3 course, but no tees, just a big patch of fairway or rough with different conditions (including one where you played out of a bunker) to be used as a learning course. If you played twice, you might hit from the left then right side, or different lie, etc. to teach you where to place tee shots, and the differences in angle, etc. Of course, shorter courses or tee sets where you have reasonable yardages left to greens after an average tee shot would accomplish much the same thing.
In any case, put me in the "keep the par 3's" camp. All the reasons are logical, but mine is intuitive - I like playing par 3 holes, and figure many golfers are with me. But, one more logical reason is golf is trying to reduce its turf footprint in the name of water conservation, so adding 2-4 more par 5 holes as suggested in the OP is exactly the opposite of what is needed.
Lastly, I agree the par 4 is the back bone of golf....the tee shot sets up a better/worse approach, and the approach shot is key. The middle shot on a par 5 is usually blah and does less to thrill or affect score than the other two, so why have it, unless two holes to possibly reach in 2 shots.....
I can't recall from any of the books how the 5 and 3 came to be? Fit the land? Simply for some variety? Probably some combo of both.