I remember the explanation for using the trapdoor was specifically for full CD32 compatibility ie Akiko. There was no other way to do it. Cost would have been very high for this reason.
This, and CD32, should have had a SIMM socket to add 32bit Fastram 2mb-4mb. Both were going to effectively be stuck at 7mhz 68020 speed which was a massive mistake for the sake of adding the SIMM socket for next to nothing.