I have never heard anything about avex trax wanting "A BEST" being her last album. Seriously, I never heard that and I kinda doubt that's the real thing.
From what I have heard, avex trax wanted and pushed Ayu to release her best-of in the last minute to compete with Hikaru Utada's "DISTANCE" album. There were many extreme and over-the-top advertisement for both albums. Including 40 different magazines photoshoots to make in 2 months for Ayu. Ayu said she didn't wanted to make this best-of album at all in the NTV interview.
"Dearest" PV is supposed to be the way she feels towards the whole "A BEST" problem.
This site explains it very well but it's in french!
http://dinibuzz.club.fr/Dossiers/Envers1.htm