That is the elephant in the room. I suspect that most of the Z cams sold have been to customers who are already invested in the Nikon ecosystem. Similarly, I suspect that the Z cams weak sales are a direct result of this. The number of potential customers is much smaller if part of your sales pitch is the ability to use F-mount glass.(...)
Of course if you don't have F-mount glass and want to start a mirrorless kit from scratch, it would be more sensible to buy a Sony at this moment. And that's the challenge to which Nikon must rise.