"As for the 21st century, I'm hard pressed to think of anyone who deserves to be mentioned or compared to any of the above songwriters."
Totally agree. Most of the best songwriters of the 21st century got started in the 20th century. Try to name a dozen great songwriters of today who are under 30,or even 40. I think most would be hard pressed to do so. I can go back to the 1960's > 1980's and just off the top of my head name dozens of songs that have become classics that people still listen to today.
But I can't think of a truly memorable song that will become a classic that was written in the last 20 years by someone who wasn't around before that. As far as the next Dylan, Cohen, Simon, Van Morrison, Joni Mitchell, Robbie Robertson....I could go on and on.....there's no modern songwriter under 40 that's even close to that stature. Do a Google search for best songwriters of the 21st century. You'll get some lists and some forum discussions and 90% of the artists named will be in their 40s or older, many much older.
Does this mean there are no decent modern songwriters? Of course not; but they're few and far between and even fewer of them would arguably deserve being called great.