I'm not convinced that Nurkic and Bam couldn't play together. I'm also not convinced that Zach is some great fit next to Nurkic. Zach is lost if he has to go out and defend all the SF's filling in as PF's because he doesn't have the mobility, at least not any that I have seen yet. And if you have two front-court players on the court who become defensive liabilities on the perimeter, you have issues in the modern NBA. And that's before accounting for the guard defense
as for Nurkic being Bam in the proverbial 'next year', sure, in some ways. Nurk is definitely a better rebounder, although some of the gap is because Adebayo plays out on the edges more. Nurk is probably better at post-up offense, although he sure needs to not brick so many shots at the rim. Nurk is also better equipped to use mass to defend post-up
but Adebayo is a lot more versatile on both ends of the floor, IMO. He's a better passer at this stage, and he's actually capable of ball-handling and dribble-drives outside in. He makes a lot of good passes off his dribble. On defense, Bam can defend the paint very well, but just as importantly he can defend on the perimeter well too. He has SF-level lateral mobility and quickness and that has high value.
Nurk and Bam are different players with different strengths and weaknesses. I think Bam has fewer weaknesses at this point, and is certainly more of a modern NBA C. As far as Nurkic improving next year, it's worth noting that he's 3 years older than Bam so I'd wonder which player has more room for improvement