My hunch is that there probably will be a future for leasing subdomains but only after majority of .com's are gone and it will be near impossible to get a reasonable domain in near any extension.
Unless you have killer domain like web.com or uk.com then leasing subdomains is far more attractive now.
Subdomains now are great for developing. If you have generic domain then you can create microsites using subdomains. For example, I have pages101.com and am planning to develop a series of microsites with a how-to learning theme. Like "art.pages101.com" or "cooking.pages101.com". Then place advertising for revenue. It is more cost-effective then owning hundreds of domains which you have to renew. Why not just create hundreds of subdomains and only pay renewal and hosting for one domain.