I absolutely agree with your general principle, as I read it: price is not necessarily determined by your cost (which goes up only incrementally with added users), but by the value (what the customer is willing to pay, which may go up more significantly).
Specifically, however, I am indicating by my post that I'm not willing to pay 4x the price, for privilege of sharing - it does not have that value to me and it's not where my expectations were level-set :).
[I currently use ToDoist which makes it easier to start (no privileges/contact sharing required) and add guest-editors/share]
My post is meant to provide feedback: I'm currently not trying their product (and I've explicitly provided why - the seemingly minor request to share my contacts), and I may not become paying customer once I figure out a way to try it (because the jump to my desired level of service is too high). It is up to the developers, if they read it, to gauge how representative I am of the target audience and therefore how relevant my concerns are; I'm assuming somebody like Pateo11 would quickly setup A/B switch - signup with and without asking for contact permissions, and see the impact :)