I agree- any changes to the weight rules in F3A will lead to design changes and an increase in cost.  Probably by making biplanes more practical than they are currently. And they would probably cost something north of $5000 a copy.
However, weight change in the AMA classes probably would not change much of anything. Right now designers are sticking  to the F3A rulebook and AMA pilots are buying those airplanes. Just check the aircraft at the top of the Advanced class. Passport, Deja-vu, Vanquish, Wind S Pro. I would be willing to bet that all four made it under 5 kg even though they were allowed 5115 grams.
If the AMA in its infinite wisdom, were to increase or do away with the weight limitation, do you honestly believe that pattern designers would optimize for AMA rules? Don't forget, the makers of pattern aircraft are aiming at a worldwide market, not just the USA. If you check pattern sites around the world you will find designs predominantly from Europe plus a fair number from the USA and Japan. Here, perhaps, we lean more towards US designs but they are not built here unless you are building from a kit and are still focused on worldwide sales..
I don't believe there is a lot of support to increase F3A weight in Europe and FAI rules are not changed without support in Europe. I would not support an increase either. I do like the idea of an allowance for the first three classes(Sportsman thru Advanced) and think the allowance should be increased to 500 gm from 115 gm,. This allows the older, second-hand, many times repaired aircraft to compete legally.
As far as changing the weight rules because they are not enforced; well, there are many rules that are not enforced. When was the last time you had your leading edges checked for sharpness. Do you really want to abolish the sound rules and have some show up with an open exhaust? Did you put a non-FAI spinner on your plane and break the 2 meter rule?
It seems to me as a CD  that the aircraft has to meet a reasonableness rule when at a local contest. If someone shows up with an Oxai pattern ship, I'm not going to whip out my scales and ruler and check it. If someone shows up with an IMAC 50 cc or even more and wants to fly Masters, we are going to have to discuss the matter but I still don't need my scales.
In summary, increase the allowances for the lower classes and let the rest go. If there continues to be a clamoring for getting rid of the weight rule in AMA only, I would make a counter-suggestion that all participants at a pattern contest sign a statement that their airplane complies with the rules.

John Gayer