Zidane's fee itself wasn't broken for a record 8 years, and that's including players who weren't just 29! By using his age, 29, you hit on a huge outlier and narrowly confined the scope of your argument.
Also, it's really rare that the best players get sold at around that age, I don't know if that's even ever happened since? If it's so rare, then of course the record hasn't been broken! If Messi, Ronaldo, Et Al would have been sold at 29 years old, do you really think they wouldn't have gone for well over £50mil? Because that's effectively what you're arguing: that players of Zidane's calibre at 29 weren't valued at more than £50mil just because they didn't get sold around that age.
The record hasn't been broken because it's so rare top players move at that age, not because they aren't worth the price.