Black suit and white shirt has never been reserved for formalwear only. Absolutely never. The recent stupidity about white shirts on this forum is one member expressing his opinion and others taking it as gospel, and the nonsense on black suits is an iGent myth. You are not approaching this from the view of classic menswear orthodoxy, you are approaching this from the view of Styleforum and iGent orthodoxy, which is a mixture of rules, some classic, some invented, and stylistic preferences- like the preference for blue shirts over white, brown shoes over black, the distaste for black in general. Threads such as the good taste thread and the WAYWRN provide a lot of very good examples of the SF house style, but they are not comprehensive compendiums of all that classic menswear has to offer. It is elegant daywear that fits in either classic and contemporary settings- it's just not within the parameters of the SF preferred aesthetic.
Not all opinions are equal. If you had just said that you would choose a navy or a gray suit, I wouldn't have cared. I would have agreed, even. I prefer those colors myself. If you had offered that you think a light blue shirt would have been better, I would have disagreed, but whatever. But you told him to avoid a white shirt, which is, by any historic rule, but any sense of color theory, by any judgement of good taste, the entirely best choice with a black suit, and the only support you offered were weakly related personal associations that you generalized to everybody else.
Pretty hard to give advice without generalising. I respect that you do not share my opinion, It's OK, I'm giving my advice, not yours, ours, or Styleforum's. But as you've raised the issue, I usually wear black shoes for a business suit, I prefer a triangular Pratt-Shelby knot to the StyleForum schoolboy standard four in hand, I think not wearing socks with shoes is a sin, and 90% of "model" fits on SF are a size too small. I agree that there is a certain "StyleForum preferred aesthetic" on many things. But a white shirt and black suit is not the best option for daywear, in my opinion, and on this issue that opinion coincides with the "iGent orthodoxy". A white shirt looks stunning with a blue or grey suit, especially for a really business-y look. But with a black suit, I'd usually leave it for bouncers, valet parkers and ticket collectors.
This really makes you cross, doesn't it? I sympathise. Every time I see jeans, cargo pants or "sneakers" in the WAYWRN thread, a little piece of me dies. :)