Summoner's potency changes (or rather nerfs) with DoTs, Ruin etc. for the recent updates were mostly to balance them with its newer kits like the Demi summons and the new Egi attacks. They probably decided to adjust those particular assets of Summoner because they must've realized how unpopular they are, which I agree with. I don't like DoTs or spamming Ruin either.

Summoner hasn't been weak at all in endgame (if we don't include 5.0 SMN). Over the years, it has always outshined Black Mage and Red Mage, or practically every other DPS since mid-Heavensward. At early levels that's a different story due to these adjustments and it's apparent during Stormblood and somewhat Shadowbringers. I dislike playing Summoner on a lower level because it felt incredibly weak without Trance or Demi summons which is where most of its damage came from. Though, they alleviated that a bit by making Egi Assaults hit hard. I wish in the next update they design it to be more cohesive so they don't have to juggle potencies around for each gimmick it has, and to not make it feel punishing to play at lower levels.

Also? 4.1 Summoner is the best Summoner.