Yeah I see your point there. It is indeed a bit of chicken and egg problem, but it's not 35% for laptop with swap-able hardware. The customization is probably the main selling point. If you want Ethernet port in your laptop, that rules out pretty much any recent model out there. Add upgrade-able memory, storage (or an option for multiple storage devices), some more specific port selection (like full size HDMI) and you might only have handful of models to choose from.
Can you do a lot of that with USB-C dongle? Sure, but dongle isn't built in and also costs extra money. Are there many people out there that don't need any of these ports? Sure! But if you're one of those that do, this might be your only real option. Especially once we get to the more exotic modules like that RGB Macropad.
I'm personally not in that market at all, but I definitely see why people might want this and saying that it's 35% markup just for the ability to upgrade and repair is missing the point a bit.