However, I agree that this doesn't affect the right to be rebooked but my take on the rebooking rules under EU Reg. 261/04 is that there is no right to be rebooked without payment of fare difference/up-fare when pax decides to be rebooked to a later date convenient to pax
I also must protest here. During this pandemic we often see that airlines are not offering any immediate alternatives if a flight is cancelled (of course, if borders are closed, this is understandable) -> so a passenger may only has the option for a full refund (a) or a rerouting at a later date at the passenger's convenience (c).
Airlines have total control over future fares. They can arbitrarily hike up rates or make cheaper booking classes completely disappear. Hence, the airline can milk the customer as much as they want. If you opinion holds up in court, the passenger protection in EC261/2004 article 8c will be completely worthless to the passenger.
Moreover, it would "legalize" a new airline business model -> selling cheap fares, cancelling the flight and then forcing passengers to pay a much higher fare at a later stage.