1. Dates are fluid. There is generally an understood fifty year spread in dates. Thus P52, our current oldest bit of the New Testament, though often cited as circa 125 AD, is more accurately said to be 100-150 AD. Why?
a. Because writing styles are hard to judge
b. Because some old scribe might write a manuscript in the style learned in his youth, thus shifting the date.
c. Because even expert paleographers can’t agree on dating, so a range is more realistic
2. Everyone (mostly) agrees Mark’s Gospel is first century, so it’s not that radical a shift, not like P52, which bumped dating of John’s Gospel back a generation or more earlier than the then best estimates.
3. There are crowds in textual criticism and paleography, among them early daters and late daters. When the consensus finds a middle ground, you’ve got something.
4. This is fun to talk about, but we know next to nothing. It’s like speculating on the next Star Trek film (and we probably know more about ST, because people intentionally leak details for publicity).