I don't know..
Usually with my experience, you pick one theme and you stick with it for a while. I don't see where a subscription would come in handy other than updates that go with the software you are using.
Well, if it was like $25 then $10 every six months the that's fine as long as it's always updated and always something improved or added. And support is very good/great
How much would you realistically pay for a highly customisable theme (almost every element customisable), with guaranteed 24h support and access to updates?
If the support offered assisted with coding errors relating to the theme and what not, then I would say $6-10/month. I don't see how that could hurt too bad in the pockets.
My mentor is also using this kind of subscription now for his themes, but with a hosting package and the package is especially set up for forum management and the support is not only for theme but on hosting as well.
Maybe if you do something like that then it will be very interesting.
I'm happy with renewable fee of $10 as I mention above. I wouldn't pay any higher then that. There been themes that I Seen over time that are like one of time payment of $35 then renewable fee is like $15 or so. Then again some offers yearly renewable fee.
ThemeHouse does offer something similar to this, but they provide access to all of their themes through their subscription service. It's not just one style. I'm not saying that there wouldn't be any interest in offering a subscription based service for using one style, but it's not something I'd personally be interested in. You never know though, especially on XenForo.
I wouldn't do that unless the project I was using it for wasn't meant to be long term. If it is a long term project, I would want to buy a theme outright, even if I wasn't able to customize it as much as the subscription-based one.