Dominion was my favourite gamemode and while I was salty and upset when they removed it, I absolutely understand why they did.
The gamemode wasn't really well designed, since if you fell far enough behind, the enemy team could just spawn camp you and you'd have little to no counter play. In those instances you didn't even have any base turrets to fall back on for defence. Balance was an issue, and later bots also became an issue, and because it started with a small player base it wasn't profitable to make balance changes to the map, which meant things got worse and the player base dropped further and further until it was no longer worth the effort for Riot.
The gamemode also in part limited Riot's creativity, since things like Azir Passive,Morde's Dragon Ghost, or Yorick's Ult wouldn't make sense at all on Dominion, where there is no Dragon or destroyed towers. Riot had to either intentionally release things that were broken on Dominion (which admittedly they did a fair bit) or they had to hold back from using those features entirely.
I spent the first 9 months the gamemode was out playing only dominion, and encouraging all my friends to play too, but while the matches were exciting and fast paced, most people just didn't take it seriously, with a lot of my friends at least saying it lacked any real strategy.
I'm sad to no longer be able to play Dominion, but I'd rather the gamemode be retired than have it be an unplayable mess.