USB-C never promised you’d only ever have to buy one cable, only that the connectors on both ends would have the advantages of higher speed, orientation flip, and flexibility.
Physics continues to demand that cables that have higher speed capabilities are shorter, or contain expensive circuitry to increase their length.
In fact, jumping from USB 2.0 to USB 3.x in a USB-C cable means the cable physically has to have 10 additional wires that are meticulously tuned for signal integrity.
You can say, oh, well for simplicity they should have banned the USB 2.0 cable, which can’t do display, but then you will have increased by a factor of 10 the price of a simple cable that a phone manufacturer or a laptop manufacturer like Apple bundles and sells primarily to do charging, and not high speed data transfer.
The fact that there are so many cable combinations is the USB standard making things MORE affordable for the average user, but more complicated to concieve of.
That’s why this notification feature is important, because it can educate the user at the moment they run into the problem about this.
A user doesn’t have to buy $20 or $100 cables for everything just in case this situation arises. They may have to acquire one when they need to plug in a display (or better yet, simply go back to the box that contained their monitor, and use the cable that came with the monitor, not the cable that came with your laptop for charging).