That isn't the case, at all. Or how come well modified series 1 E-Types can reach £250,000, when many equivalent stock examples are going for £150,000? (That is of course discounting the extremely rare, limited production models, which isn't applicable to Supras anyway, or an original condition, delivery miles car) And even now, the very nicest stock US spec Supras are only just starting to reach the money Ryan Woon got for his highly modified blue (RSP?) Supra 10 years ago.
Thats a very simplistic view, and of course, false. I've had 4 Supras, the modified ones have been every bit as reliable as stock cars. It all depends what has been done, and how.
And BTW, there is nothing "timeless" about the performance of a stock TT Supra. It was a supercar slayer when new, now many repmobiles will embarass one.
Very poor choice of investment. A well maintained Supra will cost more to own than you will gain in value, when you account for fuel costs, insurance, servicing etc. These cars are getting old, and some of the 'wear and tear' parts prices are becoming insane (priced up any suspension parts recently?) The best you can hope for is owning a car you love, while the increasing value 'offsets' some of the overall cost.