You keep repeating pace and power are two "huge parts of the game", Cannavaro proves that is just a myth, he had neither in abundance and yet was the best defender in the world for a time and is as far as I'm concerned, still the best defender I've ever seen. Mangala was just used as an example/comparison to show pace and power doesn't always translate into being a good defender.Are we really comparing Mangala with a World Cup & Balon D'or winner?
You're right, it's incredibly rare to find a CB that ticks every box, but for the price, his age, his physical and technical ability, I think he's the best realistic option. Having a stronger team & Pep coaching him could improve him dramatically in terms of concentration. He'd be a massive upgrade to Otamendi & Stones.
That's all fairy tales and optimism, you can't say Upamecano is a massive upgrade on Stones yet because right now, he suffers from exactly same lapses of concentration, that plague Stones. Stones coincidentally has worked with a stronger team and Pep and still hasn't improved in that area of his game, which completely debunks that myth too. There's no guarantees Upamecano will improve, like I said it's hope and fairy tales at this stage, he may he may not.