Russel T Suggests...
"a whole timeline bigenerated", that each previous regeneration was impacted by the bigeneration, with every 'old' Doctor now surviving his demise in a splinter timeline.
https://www.radiotimes.com/tv/sci-fi/doctor-who-bigeneration-doctorverse-newsupdate/
@CriticalCupcake I like it. Validates the Big Finish expanded universe as well.