I'm conflicted.....
While I am happy to see the ease of up-faring a ticket increase - less time I have to spend on the phone with AA after booking a ticket on their website.... It does mean that travelers who previously had no idea what up-faring was are now "in the game" via seeing a link on their reservation page and potentially taking up F space via actual purchase - which hurts from the forward looking fare cost as well as upgrade lottery perspective (not that us lowly PLT's had a fighting chance for an upgrade on a mainline flight to begin with).
But - I think overall a net positive: for those who are willing to shell out the cash, you can confirm into F and not be in the gate lottery wondering if MCE is your home for the next 2-4 hours. Hopefully the tool is flight segment specific - this could offer some good value depending on load factors and routes.
I do wonder if it will be just a straight up-fare calculation (total fare difference), or a discounted formula based on status and/or as you get closer to departure day and F seats remain unsold, or something similar to the LFBU and Kiosk upgrades of yester-year. Obviously, you'd stand the best chance at securing an paid upgrade F seat before the CK/EXP complimentary upgrade rolls happen.
Will be interesting to see how it plays out.