ChipFlip
The RTX 5080 costs $1,089.99 used. The RTX 5070 costs $529.99. That's not a premium, it's double the price for one step up the product stack, and the performance does not double.
What $560 more actually delivers
Considerably more cores, a wider memory bus, and 16GB of GDDR7 against the 5070's 12GB. In practice the 5080 lands somewhere around forty to fifty percent faster depending on resolution and title, with the gap widening at 4K where the extra memory bandwidth gets used.
Forty to fifty percent more performance for one hundred percent more money is normal for the top of a product stack. It has always worked this way. That doesn't make it good value, it makes it predictable.
The 12GB question on the 5070
Twelve gigabytes is fine at 1440p today and will be adequate for a while. It is not comfortable for 4K with maxed textures in the newest releases, and Nvidia's decision to ship this tier with 12GB rather than 16GB is the card's clearest weakness.
If you bought a 4K monitor, that's the specific reason to consider stepping up rather than any frame rate number.
Where each card belongs
The 5070 is a 1440p card, and a strong one. High refresh rates, high settings, DLSS available when you need more headroom. Most people building a gaming machine right now are best served here.
The 5080 is for 4K, for high refresh 1440p with everything maxed including ray tracing, or for workloads where VRAM capacity is the binding constraint. Buying one to play at 1080p or 1440p is buying headroom you won't use.
Bottom line
Buy the 5070 and put the $560 into a better monitor, more storage, or nothing at all. Step up to the 5080 only if you're gaming at 4K or doing GPU work that needs the extra 4GB, in which case it earns the money.