Jobs made a huge mistake by going with the 68K in the first place. DEC would prove just a few months after NeXT's October 1988 launch the viability of a MIPS-powered workstation.
Even better, in the long term, would have been to go with the 80386.
The difference is that 68k was ubiquitous, reasonably cheap 32bit capable platform with MMU that had huge availability of parts and made porting software easy. Sun was working with 68k partially because they chose it in 1980, a year after it was made available, and by 1986 they published SPARC ISA and shipped first systems a year later
Sun launched its first SPARC-based system more than a year before the NeXT launch in October 1988.
Sun came out of Stanford and was aware of the Stanford and Berkeley RISC architectures (the latter of which led to SPARC). NeXT had academia heritage, too, via Mach from CMU, but I guess it wasn't enough to persuade Jobs to go for a more exotic architecture than the one he was familiar with from Apple, or the "enemy" in Intel.
One can see a world in which NeXT goes with 80386 from the beginning, eventually pivots much earlier to software-only, and becomes a real rival to Microsoft and IBM in the early 1990s to provide a multitasking successor to DOS. Or, for that matter, IBM goes with NeXTSTEP (or just buys NeXT) instead of the AIM Alliance.
But the alternate computer history is interesting to imagine, could have been via x86 or PowerPC with IBM or something else. Same with Be.