It depends what you use your car for, really. We wouldn't recommend changing your GLC for an i-Pace if you cover lots of long motorway journeys. Equally, it might be wise to change your car if you cover lots of short journeys which aren't suited to a diesel. It's difficult to say what's going to happen to residual values for diesel cars but, if anything, the drop in new diesels being bought today might mean there's more demand for second-hand examples in the future. As a compromise, we'd suggest swapping your GLC for a newer version, perhaps with a petrol engine.