I don't know whether it's sustainable or not but the charm ran out for me many years ago.  Having been brought up on the halcyon days of the sixties that's hardly surprising.  No shortage of ferries, bikes parked side by side along the whole of the prom, and every class completely different; even the fifties were worth watching!  Mountain road rarely if ever closed and as long as it wasn't misty racing went ahead.  The bikes were real factory racers not converted road machines and overall it had a completely different feel. I can see why dry roads are essential now but I wouldn't call that progress.  As for it's sustainability somehow I can't see it lasting more than another 10 years.  The nanny state, insurance and a less than enthusiastic local population will put paid to it, not to mention the blatant exploitation of visitors.  

For anyone who wants a bit of the old TT feeling the answer lies in NI.  I've never been able to quite understand how the NW200 has become so big, clever PR over substance I suppose, but the Ulster still retains a lot of the old charm.  With the added advantage that the man in front is the leader.  Add in £84 return for bike and rider from Cairnryan and reasonable accommodation you have the complete package - just as long as it's not raining!