ST will nosedive, RS not so much. Dead platforms or previous generations typically have diminished resale >values. Remember, according to Ford nobody wants passenger cars anymore.
I used to think that.
But then when I went to Mecum auctions ... they had literally every car I've owned in the last 30 years, for often far more than I paid for it.
1969 Chevrolet Nova
1986 Corvette
1986 Camaro Berlinetta (No, it wasn't the Z28, but the digital interior was pretty cool)
1987 Buick Regal Grand National
1989 Taurus SHO
1996 Taurus SHO (V-8)
2004 Mustang GT (40th anniversary)
2005 Dodge SRT-4
2005 Dodge SRT-10 / 8.3L V-10 (Complete monster)
2014 Ford Focus ST3
2016 Ford Focus ST2 (Just got, didn't sell, won't sell)
I sold them all at a loss. (Except for the SRT-10, I made some money on that one ... ) Of course with that Viper engine, you could use the fuel gauge as a fan, so overall I likely lost money.
If I had the funding (and garage space) to keep them all, I could retire on what they'd all be worth now.
If I have to confess, the two that I truly regret parting with are the 2004 SRT-4. (Traded it when I lusted after the SRT-10, and I should have kept the SRT-4 ... loved that car) and the 2014 ST3. (But I made up for that with the 2016 which I won't ever part with). My 3rd place choice would be the 1996 SHO. Sort of wish I still had that one too. It was sort of ugly, but a great car to drive in, especially at highway speeds or on a track, such as Road Atlanta).