I don't understand that logic. It's not like Mane wasn't good enough for every team in the Prem at Southampton before he signed for Liverpool. Salah would have been the perfect RW fit for Utd (even just in his Roma form, much more productive than Mata/Lingard) and yet, he went to Pool because of Utd miscalculating or whatever.
Kevin De Bruyne at Wolfsburg and Sane at Schalke are two other examples of everyone sleeping, mostly Bayern but also all the other big guns except for City. Everyone knew those two are stars in the making and in De Bruyne's case, he was already world class. You think Barca, Real, Bayern couldn't have paid €50m for Sane or €75m for De Bruyne?
Well, when Mané did go at Liverpool, Sanchez, Hazard, Alli and Martial were already in the big PL clubs so that's maybe why they weren't interested... Perhaps he wasn't good enough for any PL team.
About KDB and Sané
Bayern and PSG did an offer for KDB. Bayern in particular were ok with kdb but didn't want to put more than the 1st agreement. Barca had Iniesta and Real was full in the midfield. KDB didn't have a place in their 11. Why would they buy him?
Sané is definitely not the same case like you said. He is still a talented young player who maybe will become class but not some kind of genius or the type of players you risk everything to have like Neymar, Asensio, Isco, Gotze or Mbappé. He was good, ok but so were Brandt, Draxler, Douglas Costa, and now are Pulisic, Malcolm, Guedes, B. Silva...
Plus:
- Real had Asensio, Vasquez, Bale, Isco...
- Barca had Neymar and players from the academy for the bench
- Bayern had Ribery + Costa and are way more complicated when we talk about 50M. Actually Rummenigge has said the club opted not to sign him "because of doubts over whether he would fit into the side and a reluctance to "stockpile" players"
2 years later I'm not sure Barca and Real have regrets. Dembele and Asensio are the wonderkids. They're probably thinking about how bring the last one: MBappé. Ribery is still top class.
Anyway we don't know if Keita can be a world class good enough for the biggest clubs. Is he better than Rabiot, Weigl, Goretza, Kondogbia or Savic ? What we know is that Barca was on Verratti, Coutinho and Ceballos last year, before Paulinho. Madrid bought Ceballos (+Kovacic and Llorente) and Bayern brought Rudy, Tolisso and now Goretza plus has Thiago.
Maybe Chelsea, MU, Tott or Arsenal were miscalculating something but it seems like the real tops clubs like Atletico, Bayern, FCB, RM, Juve were just not interested