With the best will in the world, couldn't they find any other bassline? It's tight but it's pretty much a 4 note loop.
The reality of songwriting and arrangement is that sometimes the song wants what the song wants and you just have to go with that. I think that one of the reasons this song is a pop classic is how they don't challenge the listener musically but just provide a sugary and lush morsel that delights the aural senses in a way that is nearly universal.
That said, it's kind of hilarious that he plays the exact same thing for the whole song and the one time he deviates from the progression, he fudges it up and they kept it.