I think the whole retro thing is a phase. Like 10 years ago people were throwing away all the 70's honda and kawa and whatnot's, but look at them today, a good cb 750f k1 or Z900 will go 10 grand! Right now the 60's are the period that is the hippest, with cafe racers starting to get more desired, but alas, I think in about 10 years it wont be hip to have a retro anymore, it'll just be another motorcycle, like there are people with ugly 80s motorcycles (which are getting increasingly handsome at the moment)
It seems to me that our tastes are formed fairly early in life - in our teens or earlier, into our early 20s - whether it's bikes, music, cars, or countless other things. We carry those tastes into later life, and if they happen to require money in order to indulge them, some of us can do that once we get into our 30s, 40s or later, so we create a "retro" market. I'm totally convinced that the Triumphs made in the 1960s are the most beautiful bikes ever made, which is why I have the modern equivalent, but I know that that view is influenced hugely by the fact that they are what I lusted after when I was a kid.
Fifteen or twenty years ago you could buy a good '60s Bonnie in the UK for a couple of thousand quid, but then my age group started to sniff around them and the price went through the roof. A few years later, early '70s Japanese bikes suddenly started taking off (CB750, Z1, etc.), because the young kids who lusted after them were by now middle-aged and with a bit of spare cash. And so it goes on.
These days, Bonneville-style bikes are referred to as having "classic" styling, but I think there was quite a bit of to-ing and fro-ing before people settled on that description. In fact, in the UK, at least for calculating sales, they used to be lumped in with cruisers, which really offended a lot of us. I think over time, the popularity of those "classic"-style bikes will fade, as they start to look more and more antiquated as each new generation gets older, and those of us who lusted after them die off. But "retro" has got a lot of steam in, because the idea of what retro means is constantly changing as middle-aged blokes move into the market looking for something that reminds them of their teenage bike lust. But for the same reason, it will always be a niche market, because I don't think it will ever be the predominant market for young 20-something bikers.
And there are two other points to bear in mind. Firstly, an air-cooled engine with a spine frame means a manufacturer has to produce an entirely separate bike from its mainstream products. There is very little sharing of parts, which increases the expense and risk to the producer considerably. Secondly, emissions regulations may well kill off air-cooled bikes for good in the not too distant future, and if that happens, the answer to whether such bikes are the future of motorcycles will be a very unambiguous "no".