I actually understand your point and I don't disagree that there will be a difference in density between films of different sensitivities.
But, comparing Film ISO and Digital ISO is an apples and oranges comparison.
The digital ISO standard doesn't care what the sensitivity level of the sensor is, the "developing" process is included in the standard. The ISO standard for digital cameras uses the output of the camera, specifically a JPEG image having specific qualities (essentially a digital print), to define the camera setting that created it.
The film ISO standard doesn't care about the print. (edit: yes the original standards were based on judging prints, but it is now a mathematic formula.)
If I use the idea behind the digital ISO standard and include my printing process in my film ratings (my EI) I can better compare the mediums.
I can, and regularly do, adjust my printing process with Delta 400 (to do the equivalent of spinning the digital ISO dial) from about 1250 to about 25 (first excellent print to last excellent print). FP4 roughly from 250 to 8.