Because it was worse. You were throwing away a 10% damage bonus and the ability to freely weave to be forced to weave with Ruin II because of how important Aetherflow was to the rotation. It was always a bad design decision because actually using the buff as intended cost you dps by delaying aetherflow, and it took away your ability to freely weave while still forcing you to Ruin II to ram out all the ogcd's in the opener. It wasn't that much worse because of the low the potency difference of Ruin II and III, but it was still completely asinine that the tools they gave you to solve the inefficiencies of the job were themselves completely inefficient in the opener and after any prolonged boss jump.
Being completely unable to realize that just tells me you were probably not any good with Stormblood Summoner.*
Besides, the damage it dealt is irrelevant. They could make a job with three buttons that is still perfectly balanced, or even completely unbalanced and dealing massive damage. Number tweaks are easy, you can always glue 300 potency to something and people will love it. Summoner is still well represented in savage content even four weeks in, and just like Stormblood the casual playrate will continue to increase as long as potency buffs roll out future patches.
Actually a lot has changed now, because of the introduction of Firebird Trance. They already came up with a solution to getting all your gcd wyrmwaves and as a bonus effect in my experience it drastically reduces the instances of ghosting with enkindled demi. Seriously, combining Trance and Summon makes controlling pheonix much easier and less stressful than trying to control bahamut while maintaining damage. Why they didn't just merge DWT and summon bahamut is a mystery. It mitigates pretty much every other problem except for pet ability ghosting.
Dreadwyrm Trance and Demi-Bahamut no longer exist in a vacuum. They can be compared to Firebird Trance and Demi-Pheonix.
Scholars/ninjas/etc generally don't just sit on their buffs unless the boss is about to jump incredibly soon. It was next to impossible to use "optimally" for it's design because there was never a time where you'd have it off cooldown and think "oh there's some big raid wide damage coming up in 40 seconds, I better hold Devotion for that 5% healing buff". The damage down bonus was even more superfluous and I'm willing the bet the majority of what it blocked was a mere 2% on auto-attacks. You have all these support effects tied to a raid dps increase which were largely useless because damage is king and you just hit it on cooldown like all the other raid dps buffs unless there was a jump coming. The ability was at best confused for what it wanted to be and at worst wasting the power budget on barely noticeable secondary effects.
Yes, that was the point. You couldn't use aetherflow while Trancing. Their interaction was that the two abilities are mutually exclusive and led to frustrating moments. You had to "waste" the majority of the first trance. You were left with extremely awkward timings if the boss left for a prolonged period of time like for Grand Cross Omega. It wasn't a common problem, but it shouldn't have been a problem at all.
And this is the major difference. Dreadwyrm Trance no longer provides any damage bonus. It's only use now is to freely move and/or weave. If you don't need to do either of those then you don't have a reason to need to stay in Dreadwyrm Trance. You could easily make an argument here about them removing the impact of DWT, but its purpose has completely changed. Personally this is why I would roll demi-bahamut into dwt, to give it more of that power back and to give you a reason to stay in as long as possible. Of course that would necessitate reworking Egi Assaults and demi-bahamut, but pretty much everyone wants to see some level of change in Egi Assaults so that's a bit of a moot cost.
*Do you see how pointless insults are and how little they accomplish in a conversation? They add nothing and accomplish nothing, unless you want to get really creative and call someone a ham sandwich or something.
Why not have it be instant cast by the Summoner but still radiate from the pet? This effect is already present in the game from Deployment Tactics. It's instant and radiates from the target, and this way still retains the flavour of being a pet influenced ability without the cast time jank. Also just stop killing the pet for the demi summon, that'll work great.