Summoner isn't heavily disliked; it's actually very popular because it's a real summoner now. There are people who don't like it, but it's mainly some of the former, salty summoner mains that want the old playstyle back. It's often been recommended to people for being simple if they want simplicity.

I think that Summoner has a good flow to it and is quite fun. The main issue with it is really not being able to get different summons. Beast Master will soon be able to choose what they collect/summon while Summoners can't.

A reason why they may keep Reborn mode is because every MMORPG that overhauls their combat system typically loses a significant amount of their players. It sweeps the rug from under them, changes the foundation the game is built upon (the combat) and that turns people away. This mode is the attempt to learn from that by having a changed and unchanged mode. However, I wouldn't see a logic in maintaining it if only a few players were using it, but I suspect a lot of people will use it or for certain jobs.