The 2012-2015 MBPs were slimmer than the previous unibodies but a lot of that was due to dropping the optical and spinning hard drives. The thermals were not a particular problem. Where that did become a problem was with the 2016 redesign that introduces the wedge-shaped case. The design of that would have been locked in 2-3 years earlier. From what I have read, when the case designs were being worked on Intel was promising Apple that their next generation of chips would be on a smaller processor node and would run cooler so that defined the thermal envelope of the new MBPs. Unfortunately, as we all saw, Intel’s production stalled for 5-6 years and the only chips they could produce were power hungry and hot. That caused problems for those thinner MBPs.

Apple seems to have taken that to heart when they designed the cases for the Apple Silicon MBPs and those have excellent cooling (and more ports).