I think that one can have one's cake and eat it!
I bought my 550 three years back because I wanted to own / drive a V12 Ferrari before I meet my maker (hopefully a long way off, but who knows what is around the next corner?). I also thought that it was a car on which I would not lose money: this was an important consideration, but not the primary driving force in my decision. To my surprise, it is now worth about three times what I paid for it! So the way I look at it is that if I drive it a lot, I enjoy myself a lot, but when I am not driving it, it is making me money, which is some consolation for not driving it all the time. Either way, I am a winner.
The 550 experience has been my justification for buying the Dino Dino 6934
. I bought it because I have wanted a Dino for a very long time: it is the first Ferrari I drove (way back when they were new cars) and I just love everything about it. I haven't bought it as an investment, but my belief that it will at least hold its value, and probably continue to appreciate, was my justification for buying the car I wanted to own.
In answer to the original question, yes, I believe that 550s and 575s will continue to appreciate in value, in the following order: 575 manuals the most (v. rare), 550 (all manuals, less rare) and 575 F1s the least. I hope that Mr. Sheehan is not right about Daytonas and Dinos having had their day, especially Dinos! But if he is, I shall still have the pleasure of owning and driving it and life is too short to worry too much about the money side of things - there are no pockets in a shroud!