Shaheen is a massive call as of now. Everyone knows he has the time, but at the moment his record reads:
vs Aus: 5 tests, 14 wickets, 36.57
vs Eng: 3 tests, 5 wickets, 51.60
vs NZ: 3 tests, 9 wickets, 43.77
vs South Africa and Sri Lanka, he's done quite well, and he's pummeled Bangladesh, West Indies and Zimbabwe in the limited tests he's played against each.
Yes there were extenuating circumstances for some of the above in bold (too young, Covid year, flat pitches), but he's going to have to massively improve on that record to be anywhere close to displacing Shoaib Ahktar, nevermind Waqar.