Still fairly stable at the moment but if and when a new FRS does come out I guess they'll start to drop more quickly but I don't think that they will plummet. In 3 years I bet a £20k car now will still be worth £16k.
Not everyone will be able to afford a new FRS when they come out so you will still have people in the market for the outgoing model and these people will be the reason that the prices don't plummet as there will still be a demand for that model.