It is an observable thing. There are objective ways you can determine his ability as a singer (e.g. tone, range, pitch, etc.). The only thing you presented as an example of D as a marginal singer is saying he has limited range, yet this nigga is hitting high notes all over this album with zero assistance from autotune, melodyne or any other digital processing. Marginal singers cannot do what D'Angelo does. That's not an opinion it's an observable fact. That you can't admit that or present any sort of coherent argument to dispute shows that you have no credibility and therefore your opinion on the matter is utterly meaningless.