Songwriting - Solo or Group?

For the last 15 years, my songwriting has been solo.  The songs (for the most part) have been cohesive, and exactly what I am trying to portray.  Or exactly what my inner demon wants to shout through me... 
Either way, I’m extremely proud of them.  I have written these songs myself.  I have recorded these songs myself.  The real boost has been in the mixing and pre-mixing stage, where the songs are shaped by fresh ears (belonging to John Jamieson) and become what you eventually hear.

Co-songwriting makes sense.  One person has an idea, the other builds on it, the first builds on that, and so forth.  You end up with new and exciting songs that evolved significantly from the basic chords or riff that started the whole thing.

Having four or five songwriters seems to go too far from the original seed that started the entity.  I’ve had that happen once or twice in the past, and you could actually take that seed as it originally existed and it is an entirely different song.  This just seems disappointing.

Here's a BBC article writing about the average number of songwriters in today's hit songs.