Bought Dinho when he was 28. Nice little short term solution there. Most elite CBs stay world class well into mid 30s and Koulibaly is an absolute physical specimen. That's a solid 5 years. That's not short-term in football. Besides 10 yr plans rarely ever work out in football.
Comparing apples and oranges. We didn't smash our transfer record to get Dinho. It's price that's the issue and funds which are not infinite.
Getting a 28 year old is absolutely fine. Koulibaly is one of the very best defenders there is. At the right price he would brilliant, however the club isn't going to throw outlandish sums since it's clear that's not our strategy.