"Many pop songs are now over 50 years old and have been assimilated into a particular folk community."
The folk singing tradition has been more or less moribund since th beginning of the 20th century
How on earth have fifty year old songs been absorbed into a process that is dead as a Norwegian parrot
Age has sfa to to with folk song creation - repetition is not absorptopn

"I have never made such a claim."
Others have, fairly common in modern folk clubs
"Intolerance"
The very term 'folk police" exudes accusations of intolerance - wanna guess how many times it is used

Your description doesn't even begin to define folk song - especially as Ralph McTell doesn't even sing folk songs
Streets of London comes with a little (c) which makes it the property of Ralph McTell so it can never belong to the folk; their songs are in the public domain
No cigar Dave
Jim