Why 20km?
It seems pretty pointless to make the buddy distance for legendaries 20km. Rare candies exist, and plenty of people have been stockpiling them. If they're going to make the distance that obnoxiously high they should made it so you couldn't use it at all. Not that any of this matters since I'll never get one unless I start GPS spoofing.
Answers
??? The OP's point is that rare candy does everything buddy candy does, only much faster because it can be accumulated much more quickly.
A typical Tier 4 raid delivers about 4 rare candy (I can't keep up with exactly how Niantic has been nerfing raid rewards, but that seems about right lately). If you are maxing out your 50 coin/day gym bonus even a FTP player can do about 10 raids per week (7 free+3 purchased raid passes). That's around 40 rare candies per week. Who walks a buddy anywhere close to the 800km/week that would be needed to get 40 candy for an Articuno/Lugia? Even if you managed a whopping 100 km/week, all you'd have to show for it is...5 candies.
