Grishka asks: Who is more likely to produce a good song:
-(singer-)songwriters (including teams of such, like Lennon/McCartney)
    or
-teams of specialized lyricists, composers, and singers?

How about "none of the above"?

I would think it would be by people who have a calling to say something---lyrically, musically, or both.

That is the 'songwriter'....somebody I have incredible respect for.

Then there is the 'singer'....whose job it is to 'distribute' what that person had to say.

It's rare for somebody to be great at both. Certainly there are a few.

But I suspect that many great singer/interpretors are ignored because the industry doesn't want anybody who won't also generate money by writing and copyrighting the material.

And many a great songwriter is ignored because he/she isn't a 'performer'.   Can you imagine if the great poets and songwriters of yesterday were judged totally on the basis of their quality as a performer? Would we have heard of George Gershwin, Irving Berlin, Cole Porter, many of the great classical composers, etc. Even the great Leonard Cohen (who has blossomed into a fine performer as well) would have been virtually unheard of if singers like Judy Collins, Chad Mitchell, and others hadn't discovered the songs and performed them.

Songwriters are becoming extinct.   So are great singers (in the popular, folk, country, and even jazz traditions).

There isn't enough motivation for that '3rd category' of people....those inspired to say what they need to say....to work at doing it, and for those who do, there's not enough motivation for performers to 'spread the word'.

The only solution is for all people who are primarily performers to work at finding great material....whether it be traditional, songs by non (or barely) performing songwriters, or even occasionally their own song.   And for all who are primarily songwriters to put their work into their art......and only focus on the performing aspect if you are 'called'.   Find others to do that work.

Anyway, that's my rant. Sorry if I get repetitive, but to me it just seems so obvious as I hear so much of the 'dreck' that fills up these hourlong cd's.....virtually all written by the person who is performing on them.