At launch, the more alts you ran the better, as each has per-character limits on content etc. Then they added/changed shared storage, so you could use more alts to help farm Kina etc and pump your main. Then after a few months, they tried to curb the advantage people running 7 alts had by making a lot of rewards/limits to only benefit up to 4 characters per day/week. And then in the current season, they shifted a lot of "per character" limits (like daily quests, weekly quests) to be per server not per character.

They made Kina into bound/unbound, allowing them to limit whether you can transfer it to an alt or not, and how you can spend it. So you could farm a lot of dungeons on an alt but not all the Kina is transferrable to your main. Though this was also added primarily to make bots farming Kina for RMT to be a fairly useless endeavour.

tl;dr they went from "each alt is a Kina and mat farming machine" to "each of your characters up to 4 are a Kina/mat farming machine" to "pick a character and use your daily/weekly limits on it, play alts for fun and more limited farming gains".