My apologies for repeating much of what has already been said... (particularly Vandy and Firko's most recent).
1. There is possibly too much of a good thing. I took three bikes down to Broadford, specifically to avoid spending too much time standing around (which I did a lot of in '09, when I took two bikes). While more/longer races would probably be a good thing, it would almost certainly mean that I'd only take two bikes - meaning that there would be less bikes on the start line.
2. Age races (in addition to the all-in bike age/capacity classes) also provide more chances to ride a particular bike. Again, this removes incentive to bring multiple bikes.
3. Longer races would make back-to-back races completely unpalatable. So, to avoid lynching, the organisers would have to ensure NO back-to-back races (rather than making a solid effort to avoid/minimise them, which is already a big task).
4. Regardless of how good the idealogy is, it must be balanced with pragmatism. I wonder (but do not pretend to know) whether some of the older guys will simply be turned off riding at all, if they perceived the races to be too long/hard?
I mean, we might be able to stand back and say "45 minute races are what they had in the day! This is a true test of man and machine!" - but if it results in three bikes on the grid, then have we moved fowards or backwards?
Maybe the current format has been arrived at by actually appealling to the target market, even though its idealogically impure?
Most of this is rendered moot by SuperSenior's proposal (as others, including SS50, have already pointed out).