Do you not comprehend the difference between chronological age and biological maturity?
An 80 year old Greenland shark is chronologically older than a 2 year old rabbit but that does not change the fact the rabbit is an adult and the shark is a juvenile due to lifespan difference.
Kaido was on the cusp of adulthood, Loki would remain a child for decades.
I seriously doubt Rocks was going to wait around until he was like 80 to see how Loki turned out.
In a high risk occupation like piracy, getting a meaningful return from an investment in 2-3 years is far more feasible than waiting 30-40 years.
Luffy wasn't up to much as a 7 year old either and look how that turned out.