With the number of staff on the project, you could estimate that it cost somewhere between 50-75M$. - Yes, it did.
How much revenue did it generate? - $0. Why blame Team Blue if the management decided to buy NGINX? There is some merit to buying NGINX.
How many new customers did it create or old customers did it retain? - It directly didn't. Because obviously, we never released.
How did its development or business practices make the rest of the company better on some way? - It changed how F5 has always been. Not adapting to newer technologies, not innovating at the pace of the industry. Heck, our BIGIP pipelines are built on Perl scripts. It works, I agree. But, in general BIGIP was just trying to squeeze the last bit of juice left in it.
How did they positively affect F5’s culture? - It has shown that failing is okay. Pivoting is what business is about.
Who are the future development or product management leaders who will emerge from the ashes of that project? - It is not ashes. It is the building block for future products.
One needs to understand that NGINX wasn't bought because Blue failed. Blue was the reason to buy NGINX.
I'm not gonna comment on whether buying NGINX was a great move or not, or, whether not releasing Blue was a good idea. Above my pay grade.