You might respond stating that Lufthansa did not comply with the new EU legislation concerning those flying with a mobility disability (these regulations went into effect in the summer of 2008). One of the stipulations is that the airport is responsible for supplying what is needed for you to safely deplane/board, ie, assistance deplaning. See if that gets a more useful response.

In future, I suggest that you do not leave the plane until whatever assistance you require is there. As long as you are on board the plane, a member of the flight crew must remain on board with you. Once you and your wheelchair are on the jetway/on the hardstand, they're free to leave. This gives the flight crew a strong incentive to help you get the assistance you need.