The license does not expire after 3 years, it is perpetual, but it doesn't apply to versions released more than 3 years after your purchase. If you don't want any newer features or fixes you can simply turn off updates. We warn you before updating to a version not covered by your license.
I find this to be a very reasonable license. Cost is low but provides an ongoing revenue source. I like free or perpetual licensing as much as the next guy, but us programmers gotta eat and pay the rent.
There's not much to it: we wanted steadier income and our customers wanted more consistent updates but also perpetual licenses. This model was the natural conclusion. We tried it out with Sublime Merge and then brought it over with ST4.
