I do not think it matters whether a song is cowritten or solo written. It is the quality of the song that counts. There has been some great songwriting duos both in the past and at present.
There has also been some great teams and also solo writers.
I think that people should concentrate on their own strengths. Some are great lyricists, some musicators, some are just good all rounders. It is what works best for the individual that counts. Some people are team players whilst others are loners.
The important thing about cowriting is to find people whose strengths match the others weaknesees. No point in having two great lyricists who cannot compose music or two great musicators who cannot write lyrics.