My main point is that I dont believe it is actually getting any harder to win a replica, infact I would suggest it has got progressively easier to win a replica in the classic races since Bob Heath retired.
During the period Bob was racing his superior funding and vast talent allowed continued increases in pace every year, i.e. his bike deleloped faster than almost anyone elses and he was the best rider there. Since his retirement the lap and race records stood for the best part of 10 years, while most people's bike would have improved steadily year on year. While this was going on huge sections of the course have been re-surfaced, trees have been cut down and several corners have been re-profiled, plus individual rider's course knowledge would have improved.......... Therefore I would suggest that in 2005 and 2006 a rider who had raced in the senior classic manx for 15 years would have had his/her best opportunity of winning a replica.
In 2007 the lap record and race record was broken but so was the lap and race record for virtually every TT, due in no small part to the changes at Brandish, the change here was felt significant at the TT and surely it would have more effect on a classic bike as they cannot stop or accelerate as quickly as a modern bike.
IMHO the TT riders competing at the manx are being used as scapegoats by people who are frustrated at not being able to win a replica. Show me some statistical evidence to prove that it is harder to win a rep now than in 1996 for example!