Bloomberg journalist Jason Schreier reported that Bungie did not launch Destiny 3 development due to the excessively high development cost. Some of the studio's developers are only prototyping and proposing various ideas, including those related to Destiny, but nothing has been officially approved.
This aligns with the current situation surrounding Bungie. The studio announced the completion of large-scale development for Destiny 2, and industry reports also mention possible new layoffs and a shift in the company's focus to other projects.
In other words, if anyone expected Bungie to quickly move on to Destiny 3 after The Final Shape, it seems that financial costs, rather than a lack of ideas, were the main obstacle.