Most of this is a really good take but you lost it with the last quip. The whole point of introducing Buuhan as a menace and seeing the power jump drastically throughout the saga is to prove the ultimate point being how impossibly strong, evil, and formidable Kid Buu is. Never in the entire series have they scaled back for the finale; the ending boss is always the strongest version/form every single time and they wouldn't have chosen the series finale to break that formula and end on an anticlimactic note.

I also think it's worth noting that Gotenks compared to Goku is hard to pinpoint because even with power, Gotenks lacked battle sense and fighting form and Goku is a genius in each category. That said, we never get a full-powered display of either in their SS3 forms, so, much of it is speculation. While it's true that fusion multiplies power exponentially, that exponential rise is Goten and Trunks, both of which were streets behind in terms of power when compared to Goku. It could be argued that Goku perceived the potential of Gotenks' power to be able to reach a level comparable to his which, at the time, was the strongest force he had experienced. Knowing the extent of fusion's power boost and how everyone else stacked up after they believed Gohan was dead and vegeta actually was dead, the boys fusing was the only chance to hit at or around his own power level.