^^ In the past, FIFA would say they based the allocation on previous World Cup performance, but the slots rarely changed much and the changes rarely matched up to the actual performance. I don't think FIFA makes those claims anymore.
What you suggested makes sense, but I think FIFA has made it clear they are not trying to just get the 32 (or 48) best teams, there is going to continue to be a focus on getting all confederations more spots. I just looked at the 48 allocation again so I would get the numbers right, UEFA will increase from 13 to 16 ad CAF from 5 to 9. That's a significant difference in increase in terms of percentage.
It would be a better tournament if it qualification spots were performance based, but it seems unlikely, unfortunately.