this sounds utopian. but if you didn't need the money would you go thru the hassle of sharing your car or house? the only reason people do it is to recover money because they didn't make enough to cover their lifestyle. others make it a business, renting multiple apartments in walkups and sell on airbnb creating private mini hotels. i do not know one person that has a savings and or a decent salary that would let a stranger into their home, or drive a stranger to a destination because they just love to share what they have and participate. they do it because they are fucked or close to it.

the concept of the timeshare vacation rental is closer to reality of what you speak about. shared leases, mortgages, etc etc.

a startup that divides the credit potential of multiple people into a mortgage would be a real sharing, and be able to purchase into it for a time period.

i know i need to raise a family and need a 3 bedroom house for X time. Can I effectively purchase someones mortgage for 18 years at a premium on the base apr?