I think the TT is sustainable in its current format.
Yes the weather this year played havoc and there will no doubt be people who saw no racing in their time on the island. Would I let that put me off going again?    Hell no!! 
I think the rise in popularity of social media and youtube etc have increased the numbers of "non bikers" attending the TT but i don't see this as a bad thing.
In reality bikers are an aging and declining part of the population (from my observations here in Ireland  and at NEC bike shows) and it has to be better that TT is still being supported well than to see dwindling crowds year on year.
I understand it can be frustrating to see these visitors who are simply there for the spectacle not getting into the radio commentary or the programs but when they have no knowledge of who the riders are or the radio coverage, is it surprising?

I wonder is it that some of the finer details are missed in the promotion of the tt and the mgp?
Are all visitors aware that the radio coverage is available?

A larger threat to the TT will come from the health and safety and environmental regulations being tightened.
Hats off to Garry Thompson this year. Id say the poor man didn't sleep for the full 2 weeks.