The 4070 is a 12gb card. I would not buy one, unless you have a need for Nvidia's features. The 7900 gre is similar in cost, and faster, with more vram.
While the 7900GRE might average roughly around 15% higher performance in games without RT, in those with relatively heavy raytracing implementations it tends to fall behind by a similar degree or more. And those might arguably be considered the games where performance matters most, as they need all the performance they can get to keep things running smoothly, whereas games without raytraced effects tend to be less demanding to begin with. And RT is only likely to become more common over time. So while the additional VRAM might offer a performance advantage in some titles down the line, the additional RT performance of a 4070 would likely be more beneficial in other games. So I would say that neither is necessarily more future-proof than the other.
DLSS being somewhat better than FSR at upscaling is also relevant, since if it allows the 4070 to run at a lower render resolution for a comparable level of image quality in supported games, then the 7900GRE effectively loses some performance there, potentially removing its performance advantage in many titles without RT enabled as well.
So overall, my opinion is that the 7900GRE should be priced a bit lower than it is. It would be the superior option against a 4060 TI 16GB, but against a 4070 it's a bit more mixed, and overall I think the 4070's better upscaling, RT performance, and software ecosystem for things like AI may give it the edge moving forward. AMD really needs to be offering more than the competition at a given price point to gain against Nvidia's dominant market position, but they seem to be largely content with just matching Nvidia on pricing.
As for this particular case, you would want to make sure performance isn't getting limited by the CPU, since flight simulators tend to be very heavy on CPU performance. If the CPU is what's limiting performance, then adding more raw GPU performance might not actually improve much. An exception to this might be frame-generation, where the graphics card simulates a frame between two others to help smooth motion. This can introduce some input latency and visual artifacts, so it isn't ideal for many games, but for some slower-paced, CPU-limited titles like flight sims, it can potentially substantially improving the perceived frame rates. The RTX 40 series cards support frame generation in Microsoft Flight Simulator, though I don't know about other flight sims.