It would seem to me that what the cultural district lacks is a plaza or a square; and the site of the old arena could be developed into a “Sundance Square Plaza West” with the tower becoming its most prominent feature.
As far as I know there are absolutely no plans to demolish the existing coliseum just because a new arena is being built. It can still be used for other/smaller events.
Public plazas are effective when there is a built-up urban context for them to play off of - that's why Sundance's works so well: .... There's not much of a surroundings at WRMC for a plaza to be nearly as effective - the tower and auditorium are dead & closed the majority of the year, and the other sides are parking lots. ..where you'd really get an active and lively plaza is in the places people want to be regardless of whether there is an event or not, i.e. 7th Street developments.
So I would agree with some of what you say, but not all.
As Dallas’ Reunion Arena was to its new arena (AAC), the success of the new arena was greatly enhanced and therefore ensured by maximizing the amount of event dates at the new arena. This meant that the Reunion Arena could not be allowed to compete with the new arena and would become a financial liability for Dallas. Ultimately, Reunion was and had to be demolished. Other cities have made similar determinations.
As for a plaza in the cultural district, it would have its own synenergy and would be different than Sundance Square Plaza (SSP) as it does not need to be commercially driven as is SSP. It could be similar in character as are Dallas’ Fair Park or Dallas’ Art District Plaza.
The WRMC Tower, the auditorium, Casa Manana, Farrington Field and the new Fort Worth School for Art are sufficient developments already in place to bring people to a plaza. This does not even factor in the museums, Museum Place, West 7th/Lancaster/Foch Area, UNT Health Center, and hotels that are planned for cultural district; nor does it factor in the neighborhoods of Arlington Heights and Monticello.
And then there is the Fort Worth Stock Show and Rodeo that each year brings nearly a million guests to the immediate area; and the horse and livestock events with hundreds or thousands of visitors throughout the year.
A plaza within walkable distance of all these assets weaves a compelling fabric and a compelling need for just such a project.