I’m going to out on a limb and say that a gimmick coaster is only acceptable as a replacement for Ka, but it’s necessary too.
I believe Ka was famous because of its sheer absurdity. The GP across the country who knew about Ka, and put SFGAdv on their bucket list because of it, did so because Ka was the coaster equivalent of clickbait: insanely tall, insanely fast, and a crazy-looking, unique structure. Ka stood out because it wasn’t like every other coaster in the GP’s mind — much like Volcano, it was so outlandish that it assumed its own identity in the public’s eye. It was nothing but gimmick. It was memorable.
I bet the vast majority of enthusiasts would agree that El Toro is a far better coaster than Ka was. But I would also bet you this: far fewer people in the GP knew about El Toro than Ka, and even fewer were booking a trip to New Jersey to ride it. Point being: to most of the GP, a coaster is a coaster. It takes something special like Ka to earn its reputation in the broader public mind.
If Ka were replaced by a coaster like Falcon’s Flight, or Tormenta, or Steel Vengeance, or Fury, or whatever the best “non gimmick” coaster you can imagine is, I think it would be an ill-fitting replacement for Ka. It would lack that madness, that shining beacon that put SFGADv on the map with Ka and made it stand out from all the other parks with really solid coaster lineups. SFGAdv would just become one of many parks with a really good collection of really good coasters.
Ka’s replacement needs that WTF factor. It needs to create talk. It needs to make waves and get people talking around the water cooler, states away. And just as coasters like Ka and Volcano dominated Travel Channel coverage in the early 2000s, this coaster needs the modern equivalent with social media virality. You don’t accomplish that with a slighter better multi-launch coaster or a slightly bigger dive coaster. You accomplish that with something crazy that people have never seen before. Siren’s Curse captured that spirit, I think, but it lacked the scale to replace Ka. An absurdly tall spire with spinning cars flinging riders around 400 feet in the air, and then falling back through the course backwards, not only captures that spirit but also accomplishes the near-impossible task of doing so in a way that gets close to the scale of Ka.
I for one would be sorely disappointed if Six Flags did go the “safe” route and just try to build a nice, big, solid coaster. They can build one of those anywhere. I’m much more excited, and almost a little surprised, that they’re taking the risk with something a little crazy. That’s what a true spiritual successor to Ka needs.
Interestingly, they seem to be considering GL and the new coaster areas also part of the greater "boardwalk" section. Read into that re theming, methinks. 
Management actually planned to not act as cold and incompetent as they did. 
And I agree with everything else. For me for the past 20 years, I come to GAdv for Medusa, Batman, El Toro, Nitro, and Wonder Woman. Kingda Ka, Jersey Devil, and Flash were/are just bonuses if they're working and the lines aren't too long, personally.
