Steur completed his switch on Thursday, leaving Ajax for Newcastle in a deal worth around €27m. Kieft believes Ajax have driven a hard bargain given the player's age and experience.
"That's for a youth player who still has everything to prove, and Newcastle United receives so much money annually that the total fee plus the salary is a tip for that club," he wrote.
The columnist remained sharply critical of the transfer itself, arguing that all parties may be satisfied on the surface while something more valuable is being sacrificed.
"Everyone's happy, but a lot is being destroyed here. Sean Steur most of all," Kieft wrote. "He is a talent, a big talent, but that's all that can be said. He's stood out maybe three times and been decisive twice. You've still got so much to prove at Ajax and in the Eredivisie."
Steur is reported to have pushed for more first-team minutes at Ajax, a demand Kieft dismissed outright.
"You can't come with a request like that," he said. "Not in the past, and not now." The former striker argued that in an earlier era, such demands would have led either to a player leaving the club or being demoted back to the youth ranks.
Kieft reserved some of his harshest words for those around Steur.
"It's so shamelessly brazen," he wrote. "Especially knowing they can cash in elsewhere along with the player. Clubs then feel forced to sell just to get some kind of financial compensation before such a talented player walks out the door for nothing."
The 63-year-old has little expectation that the midfielder will succeed in the north east of England.
"Steur is doomed to fail at Newcastle United, because it's not just about how good you are with a ball there," he wrote, pointing to physicality, pace and duelling strength as areas Steur already struggled with at Ajax. "What does a teenager have to look for at Newcastle United then?" he asked. "Steur is no Lionel Messi, Cristiano Ronaldo or Kylian Mbappe."
Kieft closed with a pointed piece of advice for the youngster.
"You should have given yourself another two or three seasons to develop at Ajax in the Eredivisie," he wrote. "Then someone like Steur could still have made a wonderful transfer later and cashed in financially."