I don't think you need to have it match the depletion rate. Even at say 10kW you'd doubling the effective range. The point is people want to be able to use their car for that twice a year road trip, or drive to the inlaws for Thanksgiving. If you can attach something that extends the range for these occasion then that's probably enough.

Furthermore, the generator can run 24/7, but realistically you will at most drive ~6 hours a day. So for a leisurely road-trip this may be enough to avoid needing stops at charging stations