A single RTX 4090 can run at most 34b models with 4-bit quantization. You'd need 2-bit for 70b, and at that point quality plummets.
Compute is actually not that big of a deal once generation is ongoing, compared to memory bandwidth. But the initial prompt processing can easily be an order of magnitude slower on CPU, so for large prompts (which would be the case for code completion), acceleration is necessary.