Thanks for your post, there is no problem at all with putting these subjects up for debate.
Just to clear up a couple of points, the size modifier (SM) does indeed dictate the amount of points that a reported scenario attracts, to arrive at the correct SM you actually cross reference the file size (in KB) with the amount of turns, this is somewhat crude but does reflect the size of the scenario (in units) and the turns to be played.
The bonus for completing a campaign game is 25 points, a campaign game is defined by any scenario of SM8 or greater (the SM goes up to 10).
If I had been around 22 years ago when the Blitz was launched I would certainly have asked for a scenario reporting system that took into account of the actual turns played, of course this would be reliant on players being honest when reporting, however the guys that coded the scoring system decided to go with the system we have now (perhaps anything else was beyond there ability to implement?) and that has underpinned the vast amount of results we have accumulated in two decades, even if we had a wonderkid webmaster I am not sure they would be able to retrospectively change those thousands of game reports if we tried to implement a new scoring system.
So although your idea makes sense, the reality is that the scoring system is what it is.