The upcoming Z could definitely be made available in a convertible, 95% of the engineering was already done and manufactured for years on the Z34 chassis.
Blame the pandemic.
Without a pandemic constraining production and leading everyone into hyper inflation, Nissan would've made more than 2 Z trims available for first year, sales would've exceeded expectations leading to a convertible offering for the 2024MY.
However, it's going to be years before all car manufacturers stabilize, and in the interim inflation is going to tear everyone up. It was smart for Nissan to limit the Z to 2 trims, it's the healthiest way going forward.