Logically, it makes the most sense to include a 45+ 3/4 or 45+ 3/4/5 field and eliminate the 45+ open field.
Why? Because 45+ Cat. 1 and 2 riders are good enough to compete to win in the 35+ 1/2/3 field. Any disadvantage of age (which is arguably none at all) is overcome by the advantage of competing against a field containing 3's.
As it stands now, the 45+ 3's are getting the proverbial screw. The 45+ 4/5 riders can (and generally do) compete in the 35+ 4/5 field. But 45+ 3's have to choose between competing against 1's and 2's in the open 45+ cat or in the 35+ 1/2/3, or against all comers in the Elite 3's.
Thus, the most fair solution is to include a 45+ 3/4 or 45+ 3/4/5 field and eliminate the 45+ open field. Isn't that simply logical?