Emma Raducanu hadn't played much "top flight" tennis before she hit it big. Sometimes it happens. I've got no idea if this lad is worthy of that analogy, but you definitely can't rule out his worth solely based by he's only played 25 Championship games.
Some players will burst onto the scene and become stars overnight and stay that way. Foden, Saka, Rooney, Owen, Beckham etc etc etc. Many others have huge potential which for various reasons never quite materialises or burns brightly for a short period and is never recaptured.
Nobody knows whether Raducanu will be winning Grand Slams regularly over the next decade, might just bump around on the Tour creditably without getting near another major final or decide in a couple of years that the strains of touring and playing tournament are too much stress (see her withdrawal from Wimbledon) and instead go off and become a mathematician (she managed an A* in A level Maths last summer while playing a bit of tennis!).
Johnson might be a Rooney or Foden who will tear up the PL. Or he might not. Even with all the stats in the world we don’t know. So it comes down to how big a bet on him somebody is willing to make. Will Forest gamble on holding him unless they get £15-20m? Will we gamble on meeting that price? Will he kick on to be worth that and more or will one of us (or another club) get left holding a player who ends up a McEachran of unfulfilled promise? It’s easier for Forest to gamble this way, if Johnson doesn’t progress with them they don’t lose much from a player who cost them nothing. If another club with limited resources, like us, gambles and loses, they’ll have spent Toney+Maupay+Benrahma+Watkins+Mbeumo to get less than one of those players. I’m glad it’s not my job to make that bet or my money that’s at stake!