The 2004 Accord has a DIN slot in it. Which has a myriad of options available for it, despite the dwindling aftermarket.
Have you seen how to upgrade a 2023 Honda Accord infotainment system? You can't without major work. You can use the factory head unit, and feed all the amplified signals into a $1000+ sound processor, with a bunch of other modules specifically built for the car, then run the speaker outputs out to some crazy-ass 8 channel amplifier (because more speakers means the stereo is better for some reason), then feed that back to your speakers. And then at that point, all you've upgraded is the audio, not even the head unit itself. And why does it even have a center channel again?
Also, people obviously care about what infotainment is in their cars, as there is a huge amount of people saying they won't buy a car without CarPlay. Sounds like people DO want to augment their infotainment systems. Nobody is trying to do it, because you can't do it. Imagine if you could buy the car you wanted, and install your own accessories in it.