I'll correct two things here (though I agree with the general point) :Monaco once got relegated trying to do that. It is definitely not a fool-proof system. The intrinsic issue of being a Southampton is that every signing you make is a big gamble. Leicester is trying to do it and has spent a ton of money on busts. Benalouane, Mendy, Musa, and Slimani are good examples. They signed the right players a handful of times, but truthfully, they're straight back in the yo-yo zone where they were before. So they went straight back to signing 28 and 30 year olds after spending all that money on busts.
1- Monaco didn't get relegated with this model. They got bought in Ligue 2 AND THEN applied this model. In fact, they were close to relegation FROM Ligue 2 when they got bought. First purchase I think was Ocampos for 15M€. Which proved to be a bust
2- N. Mendy isn't a bust by any means. He got injured as soon as he started playing for Leicester. He's still injured, but the talent is there.