It wasn't the danger that was the driving force for the boycott, it was the compulsion that riders felt to compete, either directly from their employers or in their own minds to support their championship ambitions.
The TT course was not especially dangerous when compared to plenty of other championship courses, Imatra as just one example. It's problem was that it took so long to learn. Nowadays newcomers spend months in advance learning their way, which just wasn't an option for riders competing in the world championships.
A boycott is not going to happen today, as long as safety standards are at least maintained, because all competitors are now truly willing participants.