Shadab is about 25-26 in reality, and he has only played 11 domestic FC matches. There lies the problem.
Leg-spin takes years and years of grind in the long format to develop unless you are some prodigy like Shane Warne.
As far as him developing into a world class leg-spinner is concerned, well that ship has now sailed.
He got a very early taste of the T20 bling and shot to fame on the back of PSL 2. He never focused on developing his skills for the longer formats and now he is getting exposed in the shorter formats as well.
PCB is trying hard to mold into Yasir’s successor, but he is not interested in Test cricket and he will never be half the Test bowler Yasir is.
Agreed, the early exposure to PSl and T20 leagues was the undoing of what could have been a great bowling career for Shadab.
You look at his performances around 2017-2018, and if you watched his bowling, he was quite threatening. He got the ball to spin, he had the variations, and he was attacking. Yes, he used to get smacked sometimes (Hardik Pandya's notorious bashing of Shadab), but there was a lot of potential there.
The thing with Shadab is that his action is quite repeatable, and that's a good sign for a leg-spinner who wants to succeed in tests. If your action is repeatable, it means that you focus more attention on the other side of the pitch. Similarly, he had good control over his variations, though he might have been impatient with how he used the googly.
If he did not venture out into T20 leagues and kept playing FC cricket, we'd have someone almost ready to hold the spin-bowling front alongside Yasir, but instead we have a player who is equally as confused as us about being a bowler or a batsman.
Shadab threw away a golden opportunity to carry an unchallenged spot in the Pakistan setup, because our selectors would look at a leg-spinner averaging roundabout 30 rather than a finger-spinner averaging 25.
Right now, I don't see anyone in Pakistan capable of being labelled as Yasir's successor. Zahid is too uncontrollable (ER of 4 in QEA is poor to say the least), Qadir hasn't played FC Cricket, Shadab hasn't played FC Cricket, so there's not much we can really say.
You go to finger-spinners, and there aren't any good youngsters either.
So realistically, Shadab has lost the chance to develop his bowling because he's lost those years behind him. If he's serious about bowling, he needs to drop the bat.