Its not only textures, but 3d models etc. Textures actually do not take too much memory nowadays, they are supposed to be compressed anyway + people are generally moving to procedurally-generated textures.
The question is rather: does any game environment actually require more then 1GB of data at a same time? Bad gaming engines (which just load everything at once) can pass that limit rather fast, that is true, but a well-written game will only load the data into VRAM which is likely to be used soon.
For instance: assume that 500MB of the VRAM is reserved for some internal stuff, like framebuffer, shadow maps, post-processing effects etc. This is a rather generous estimate, as the complete framebuffer of a 2560 x 1440 screen would actually take about 50Mb (more if the game uses "true" HDR).
This leaves 500MB for game data. If the complete data of a modern game takes 20GB of hard drive space, and the game itself comprises of 10 levels, it would mean that our GPU can "fit" half a level at each time. This is more then is actually needed.
Increased amount of VRAM would help with either badly designed games or games which have very high level of detail. The question is, still, whether the GPU itself would be powerful enough to render such games at iMac's native resolution.
P.S. This post is entirely speculative, it would be interesting to see some actual benchmarks. I predict the difference will be negligible 😉