I guess in the broadest possible definition, all internal combustion cars will eventually become classic. Like for instance if someone sees an original Game Boy, they will say "oh, what a classic!" Or it could mean a classic design, to which all 911s are classic. However, within the OP's question, whether it is of investment grade or whether someone looks at a 996 and does not just think old car but desirable classic car, that's maybe another question.
My father had one from new, (tiptronic, all wood inlay options ticked). He paid full wack back in 1999, and it was part of the family for 15 years. I bought mine (manual, next to no options - just the way I want it) with more overwhelming positive personal experience than negative market opinion. Its funny because back then it was an expensive, high-end, desirable, performance car and now it's tryingot shake off it's bore-scoring, IMS, odd-ball, "its worth less than your Fiesta mate" kinda car. Back in the day the 993 Turbo was still being produced (just about to be discontinued) and I said to my dad "why don't you get that one" (was in the same catalogue back in 1998) to which my dad said "I want the new car, not that old fashioned one." Back then the 996 waitlist was over a year and wherever we drove, the 996 was fawned over. Oh how times have changed!