What would have happened if John Lennon ended up playing bass?
You can hear the answer by listening to ‘The Long and Winding Road’, on which he actually did play bass.
I will simply quote Ian Macdonald’s assessment, from p. 339 of Revolution in the Head:
Recurring wrong notes at 0:28, 2:10 and 3:07; mis-strikes ar 2:39 and 2:52; drop-outs at 2:59 and 3:14; a fumble at 0:19; a vague glissando at 1:03; a missed final push at 3:26.
I’ve just listened to the song, and these are only some of the mistakes Lennon made. As Macdonald points out, at 1:59, as McCartney sings ‘You left me standing here…’, you can hear him grinning at how badly Lennon’s playing.