1440p 120hz is not the same amount of pixels as 4k 60.
1440p is 3,686,400 pixels.
2160p is 8,294,400 pixels.
If you multiply by 2 (to represent the 1440p at 120hz) you get 7,372,800
Still almost 1 million pixels shy of 4k.
And, strictly speaking, that isnt how GPU horsepower is utilized anyway. 1440p 120fps still takes quite a bit less GPU horsepower than 4k 60, even accounting for the disparity in pixel count
You dont need a 3080 for 1440p high refresh. It will likely last longer without needing an upgrade, but it is't necessary. Particularly if you're OK with turning down the settings that almost every AAA game has that add almost zero graphical fidelity but crush GPUs. And given how well DLSS 2.0 seems to be doing, its likely to actually get easier as more support for that comes in.
Besides, you might actually be able to buy a 3070. You're not going to be able to reasonably expect to get a 3080 until February at this point.