I find this point intriguing, I'm genuinely curious how this could add a lot of costs.

Specifically, I'm not asking for a full blown dedicated port (like Apple's Lightning port, or like laptops accessory bays), but just a few apparent contact points that a tinkerer could tap into to access one of the buses of the phone (I2C, USB, whatever is availble).
I was (perhaps wrongly... please update my knowledge !) that this shouldn't be that much complex, either just leaving some traces visible from under the battery cover, or maybe the additional cost of pogo-pins for convenience (they didn't seem that expensive when they were a must for XBox haxking), maybe at worst a chip to buffer voltages (not necessary for I2C as far as I know).

I was under the impression this doen't raises to much the costs from a hardware point (no custom mechanical port, just a few traces and pogo pins) nor from the software (the kernel already has a driver for the I2C bus for the sensors).
I would be work for the UI/UX habdling them, but that's basically the work foe the actual tinkerer building stuff on them (see drikv and kimoli's work on Jolla TOHKBD).

I'm specifically NOT interested in something like Jolla's "The Other Half" NFC platform. That would require extra chips (NFC radio) and extra software (the whole platform for automatically detecting OtherHalf's NFC, downloading corresponding software packages - like drivers or ambiance, etc.)
That's tons of work for just a small non-essential feature.