It's exactly this. The rotation is already over 2 minutes long, so adding more to it just means more bloat. They need to trim back the job substantially so they can develop it further. Adding another trance just means, as was said, extending this already very long rotation even further, when it already has one of the longest rotations in the game.
This being said, it's too early to tell what they have planned for summoner, they only thing we know is that egi glamors had to go on the backburner because any changes they make now would not be compatible with the plans for the job in endwalker.
So this could mean that skills like egi assault are one and done summon animations and that your carbuncle or whatever else doesn't follow you around anymore. This would be easier for them to balance than a traditional pet class and more in line with most final fantasy games where summons are just spells rather than controllable creatures. But it also doesn't mean that egis are also going away entirely either as they may serve some other purpose or be brought about in the code of the game differently so that it would just cause more work to add more glamors now that then have to be recoded when the job is reworked.
