As someone who has lived alone for eight years using only public transport in an area with excellent public transport, I can tell you that you are both right and wrong.
You are right in that if there is just one bus line, then it would only serve a small subset of people in this photo.
But if you only make one bus line then the public transport system is doomed to fail.
A good public transport system will have multiple lines converging to the same interchange, and in the opposite direction it will have multiple lines departing the same interchange, following the same route and branching off when needed, this way you have added capacity and redundancy at the start of the line, and it gets reduced as the need is reduced.
Then add lines that are circles in higher density areas, this means that no matter the direction all passengers can get on all departures, you csn also quickly add capacity by adding busses that goes in alternating directions.
All of this means that travellers can define their own route along different bus, train, tram, metro and ferry lines.
Public transport is not ment to be point to point, it builds a framework where people decide what parts the want to use.