I do like the clean esthetics of a sport in which the players step onto the court alone, and are entirely on their own during the match. I'd have no problem telling coaches to sit in the cheap seats. I'd agree that huddled sideline conversations shouldn't be allowed to slow the pace of the game. (I've come to hate modern basketball, where it can take 15 minutes to play the last 15 seconds, and a half hour if it's an ACC game.) But that's an esthetic judgment. Why does it matter in tennis?
Neither can I.
I saw the video of the coach. He admitted that he had been coaching from the sidelines, but he said that it was commonplace, and that he had never been penalized for it in the past. Whether Serena saw him or not, and whether seeing his signaled advice would have affected her play, I don't know.
Rarely mentioned in all this criticism of Serena is that after she lost, she immediately approached the net and embraced her opponent. That sure looked like losing graciously to me.
I think what it boils down to is that the coach does not have to focus on, for example, actually catching up to and hitting the ball, allowing him (in this instance) to focus on strategy, positioning, etc., and then relay that info. back to his player. That makes the effort sort of a “two-on-one” attack on the opponent.
Now, I would have no problem if coaching is allowed on BOTH sides of the net, but I guess tennis is supposed to be “pure” one-on one. If that’s what the organization sponsoring or regulating the match wants, then it’s up to them, not me. If somehow that makes matches less entertaining, maybe I won’t watch, but in fact it makes little difference to me.
The interesting thing in this case is that Serena’s coach claims the opponent was getting coached too. Maybe Serena’s coach was just more blatant about it? I’ve not seen video addressing this point or making a comparison.