John Winston Lennon would be 80 years old now. The Beatles might have reunited multiple times. He might have recorded a dozen more solo albums. He and bandmate Paul McCartney might even be touring today, as the greatest songwriting duo of all time.
All this and more, had Lennon not been slain in front of his New York City apartment forty years ago tonight (at the very moment—10:50 EST—that I publish this). For me, that ended The Beatles.
Counterfactual history is by definition a speculative exercise, so you might say I'm a dreamer. But I would argue that The Beatles were a lock to get back together. They had already considered it several times in the '70s, but according to McCartney, all four members never agreed at the same time. "One of us would always not fancy it. And that was enough, because we were the ultimate democracy.”
For business reasons alone the pressures would have continued to mount. Multi-million dollar offers had already been pouring it and would have only increased. Even if none of the four Beatles personally needed the cash over the next twenty years—which seems unlikely—eventually they would have caved to the demand to come out of retirement to raise billions for some worthy cause. (In this scenario, the end of The Beatles would have come in 2001, when George Harrison died.)