Tomcat you need to check the regs. Current TT riders and past Manx winners are allowed to ride in all Manx classes except the Junior and Senior.

A few thoughts, TT riders have always been allowed in the classic Manx races and have pretty much always dominated them. The last people who I can think of off the top of my head to win Senior and Junior classic races who weren’t racing the TT at the same time are Graham Rhodes and Derek Whalie.
If you banned current TT riders from the classic races there are no races you would take 10-15 riders out of each race straight away.
If TT riders were banned would other riders come back to fill the gaps?

Personal opinion there is no problem with the TT riders the problem is with lack of continuity in the eligibility rules and application of them equally to every machine.