You're speaking the truth, but more producer involvement isn't necessarily the problem. Frank Sinatra, Elvis, many classic country stars, and others would sing and do little to no song writing or instrument playing. Ice Cube wrote lyrics for Eazy E and Dre did the beats. If Eazy did both of those for himself then the songs would have been worse. Def Leppard and other rock bands wouldn't have been the same without help from producers like Mutt Lange. Michael Jackson had Quincy Jones, the guitar solo on "Beat It" was by Eddie Van Halen, etc.
Teenagers tend to have the most interest in new music. Because of this the major labels over the years focused more and more on them. In the 1970s when they signed someone like Elton John or Billy Joel they knew that they would have some appeal to teenagers, but that also people in their 50s might be able to get into them.
If the labels back then had the attitude they have today, they may have said "This Elton John guy's ballads sound like old people music. I'm not sure how appealing this will be to teens. I guess we can't sign him." Or they would have told him that they would sign him if he used less piano, less lyrics, repeated the chorus even more, or something like that.