Cost. Why include a tech no-one's really using? That's why the parallel port was removed from the old PS1, and rightly so.
Any real GT fan uses a wheel. Map accelerate to the trigger and be done with it. It'd be stupid to include more expensive analogue buttons for 0.001% of the game library that values it. As you say yourself, most people probably aren't even aware the buttons are pressure sensitive, showing how pointless and underused that feature is. Only if the pressure sensitive buttons are negligible extra cost does it make any sense to include them.
All true GT fans own a wheel? OK sure. How many "true GT fans" are there? Not 10 million, by your definition.
I'll list some games that use pressure sensitive buttons off the top of my head, and then you can calculate whether or not it is 0.001%.
Metal Gear Solid 2
Metal Gear Solid 3
Gran Turismo 3
Gran Turismo 4
Gran Theft Auto 3
Grand Theft Auto: Vice City
Grand Theft Auto San Andreas
I could continue, but pretty much a huge proportion of driving games on PS2 used pressure sensitive face buttons for braking and acceleration as well, not just GTA. We are also forgetting the point that more DualShock controllers are sold than consoles, and others are severely overrating how much of a cost burden the DualShock is for Sony (re: it isn't a cost burden).
And why include the tech? Because it's an option there for usability. It can be adjusted for different games. The digital on-off threshold can even be adjusted. If someone wants to use it, it's there. It's not just about the bottom dollar, because frankly the DualShock is a money maker. It sells more than the console itself, and a profit is made from it on day one. There is little reason to look to the DualShock for cost savings. Whatever savings made from taking out pressure sensitivity are guaranteed to pale in comparison to choosing off-the-shelf components rather than continuing with Cell.
The DualShock is a great controller for a million reasons, and having this extra sensitivity that other controllers don't have is just another reason it's such a versatile controller.
The truth is we don't have a good enough reason that Sony will take it out, and I don't have a good enough reason Sony will keep it in. So let's leave it at waiting for the official announcement rather than acting as the armchair president/CEO of SCE.