I know the main connection is a real pain, but I begin to conclude on some interesting facts. I might be wrong, but this is a observation.
When demolishing a main connection, you have to build a bypass first. Some times it works, other times not. It is easier to build a smaller bypass on the main connection itself than building a completely new bypass from town to town.
When building a bypass from town to town, the bypass must be build in such way the travel time will not be longer than the main connection, but shorter. This means a faster bypass or shorter bypass. Therefore it is advisable to use a faster country road or the same one from which the main connection is build with.
I think the defining rules for preventing demolishing the main connection is the ability for the game to assign destinations for the sims living in the town. If it use the preferred connection to reach the destination, it will assign the connection as the “main” connection.
If you build a bypass and the game is able to use the new bypass for it has shorter travel time, the main connection will be nullified. Then you can demolish the original connection.
I also noticed people have problems with a bug where a person is unable to demolish a street segment without it touching any main connections or etc. This is a bit sad. 🙁
I think you have to demolish all the buildings alongside the segment to be able to demolish the segment, but I don’t know if it works.