Why stamps that say ‘1st’ stay valid in perpetuity (so hoard ’em)

Stamp prices will be hiked on 30 April, so bulk buy "1sts" and "2nds" now and they stay valid so you can beat this. I’ve been yelling this to anyone who’ll listen, but some have questioned whether this will always be the case. So I thought I’d explain. What’s happening? From 30 April, 1st class letter-sized stamps...