Can we go back to the original questions... as so often, they get lost in the mix
1.will commercially popular song,[eg songs written purely to make money] ever erase songs written by people who write purely for fun.

How can they be erased... question makes no sense at all. Something written just to make money does not automatically become popular, and therefore not commercially successful. A Song has to be liked by a lot of people before it becomes popular.... it has to be purchased by a lot of people to be commercially successful..
2.do commercially popular songs songs become folk music if they are sung on football terraces and adapted by singing sports fans.
No to that one ....
.Surely.. Most professional writers do so with the notion that they will be commercially popular. (earn a living ).. The best of them make the songs they write sound as though they are heart felt !!!!!They make a connection....and its seamless seamless Nothing wrong in that Dick !!. So cleverly done you would never know !!..