There will always be more data that could be relevant than fits in a context window, and especially for multi-turn conversations, huge contexts incur huge costs.
GPT-4 Turbo, using its full 128k context, costs around $1.28 per API call.
At that pricing, 1m tokens is $10, and 10m tokens is an eye-watering $100 per API call.
Of course prices will go down, but the price advantage of working with less will remain.