Apple and TSMC knew N3B was not a long-term process. Apple went to it because they needed to validate 3nm SoCs and TSMC needed to validate "larger scale" N3B production.
But that general demand could be an issue. Apple was able to buy (effectively) all initial N3B production because nobody else was ready (or willing) to go to it. N3E is the process "everyone" wants to be on and while Apple can buy a majority of it due to their "Best Customer" status, they likely cannot buy all of it so they will need to balance capacity TSMC is able/willing to sell them with their product portfolio. And we know A18 and A18 Pro are going to be the most-important followed by the M4 family, starting with the base M4 and then M4 Pro and M4 Max.
What
@Harry Haller said.
Apple has traditionally been much slower to update their desktop line, partly due to lack of CPUs from partners (Intel) and more partly due to them making up a relatively small portion of Apple Mac sales (15-20%) due to the longer replacement cycles for them.
I mean let's be honest, how many current M2 Mac Studio users are going to replace with M4 if it did drop at WWDC? These people spent $2000 to $9000 on their machines only one year ago and they are still pretty darn powerful, even if they are not the most powerful.
Apple did do a relatively decent review of the M4 and they will probably talk more about it's AI-specific features at WWDC when the announce macOS 16. And then we will have the technical deep dive of the M4 Pro and M4 Max this fall when the MacBook Pro (and Mac mini?) launches.