For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
Please see our announcement here.
I really, really, really hope that what you predict, kopchinsky, will never be a reality. I wouldn't know where to go or how to start over if the Fabrik I know mutates to something I can't use any more. It's taken me (literally) years to grasp Fabrik, and apply it to our needs.