One of my assumptions is getting off earth will always be the most expensive part of any space activity. Implications are

• getting bulk materials from other than earth will be critical to make it sustainable
• some degree of local manufacturing and farming is vital
• mining in space will never be worth returning to earth

Therefore even relatively small permanent presences in space might quickly turn to “trade”. If we can refine mineral X on the moon it may be cheaper to ship to a Europa base than for Europa to find it locally, and both be far cheaper than bringing it up from earth