Modern Kratos wouldn’t be nearly as impactful or enthralling if we didn’t intimately know his past and what he is capable of. Replacing him with another character who acts identical and had a similar background revealed in flashbacks would just undercut how Kratos acts now.
We see him show restraint his younger self was incapable of, and how when hes holding back, its not for his benefit, but for those who are antagonizing him and his friends. HE knows he is a monster, doesn’t view himself as redeemable in the slightest, but has no intention of returing to his old ways while he has the ability to help those he’s come to care for, and also show his son a better a path than the one laid out for him.
So while yes, ps2/ps3 kratos had all the depth and bredth of a puddle, modern Kratos is built entirely off that puddle and wouldn’t hold its own weight without the previous foundations.