They have a statue of her in front of the WB offices, why would they drop her?
She's 38 years old. I think there's some consideration to the longevity of the Wonder Woman character. RDJ and ScarJo look appreciably younger in the early MCU days than at the end.
From an industry standpoint, from a marketing standpoint, privately and quietly, there are some basic stereotypes of performers and aging. For example, as male actors get older, having longer hair helps them look a bit younger. Somewhat. Also certain body types and facial structures are scrutinized to project their practical longevity.
The shorter you are, the more pronounced any weight gain will show up on the person. Adding 15 pounds if you are 6'4 is one thing. Adding 15 pounds if you are tiny like Anna Kendrick is another. The common rule for women is you can "Look Old In The Face Or Look Old In The Body" You can stay very thin, to Hollywood standards, but then your face also thins out, showing age very badly in some cases. You can put on weight, which hides your age a little bit in your face, but obviously you start to loose the thin body that is considered the industry archetype in most cases. And that "pudgy" kind of look naturally makes you look older.
From a projection standpoint, someone like Lupita Nyong'o is going to age better than Amy Adams, against a relative baseline. Things like melanin, collagen, bone density and dermis all have some trends based on ethnicity. In effect, someone like Amy Adams will not have some of the natural advantages that Nyong'o has with her skin. Can Gadot give the DCEU a new trilogy as Wonder Woman under James Gunn? Three films could take 7-9 years. Gadot is beautiful. Stunning beautiful. But will she hold that into her 40s? I don't mean against a real life person baseline, but as the lead star in a major film baseline. The first Wonder Woman made money, but the concept was seen as bankable, not Gadot herself. I don't wager most fans will go see a film, on any premise, just for Gadot, like they would for DiCaprio, Hanks, Denzel Washington, etc.
The age issue is pretty brutal for women in any aspect of "celebrity" life. It matters how good looking you are, how thin you are and it matters how young you look. Then it matters who you sleep with and who you are willing to sleep with to get ahead.
Gadot is also Israeli. I am not going to get political here, but to say that some current events put her in a lose/lose/lose situation career wise. If she says nothing about some current events, that would be held against her. If she said things one way, it would be held against her. If she said things the other alternative way or even a "middle ground" way, it would be held against her. It was a lose/lose/lose proposition for her. Which may impact back end financing on some future projects where she might be involved. Something to consider is the industry is not exactly short of those with a Jewish background. Again, another complexity in play. Put it this way, none of that, just from a career standpoint, is going to help her any.
Put it this way, James Gunn has to deal with entire tiers of complexity that don't hit the public eye. It's just not that simple as to say many fans love Gadot as Wonder Woman today. I know Gunn, like Snyder before him, and like Feige, will have to take some public hits for unpopular moves or things that fail under their watch. But it's just often not that simple. However, to be fair to fans, they don't care about that and they don't have to and everyone understands that. Fans want something great, fun and entertaining, part of paying the price of "admission" is also buying the right not to hear why others can't deliver.