ENS has become substantially more useful, and the US dollar has declined substantially in purchasing power since ENS was launched. Both of these point towards a higher price being warranted, and in this instance a higher price serves both the primary goal of regulating registrations and the secondary goal of generating income for the DAO.
Incentivizing longer registrations has the opposite effect - it incentivizes registrations by end users over speculators, because there’s little to no correlation between registration duration and sale price. Speculators are overwhelmingly likely to register for short periods in order to minimize their costs, so they do not benefit from tiered discounts on longer registrations.