I have paid the 3-5 year premium for lifetime membership for 4 product groups (2 themes and 2 plugin sets) and in the first year logged 1 support request. I have purchased the lifetime membership because who knows where they will be financially in a few years and this way the sites these products are used on are more future proofed. Yes, in every case the theme or plugin would continue working after the first year, but since updates for security and WordPress changes are required, that is only a short term reprieve.

I can see the point that support costs might get out of balance. I wouldn't have a problem with lifetime upgrades but a year of support. Make adding additional support terms affordable in case they are needed later on would assure me I could get help in an emergency and keep support from becoming a burden for the company. There really are lots of options for pricing and support. It does not have to be either or.

For what its worth, I often find solutions in the forums from other users and previous posts. Good plugins and themes are often featured in blogs and how-tos. Building a community of happy users, good documentation, and making support forums and documentation easy to search also helps.

I'm on my way to checkout ThemeFurnace.