When Agile came about to company (large American corp) I work for, around 2015 (arguably quite late), I was quite skeptical. In my opinion, a decent waterfall (basically a sort of compromise) worked pretty well, and I didn't like fake "innovations" like Scrum or renaming everything in project management terminology.

Then I read Steve Yegge's Good Agile, Bad Agile. It basically says, Agile is just a Kanban queue. And I think I got it, and I think that's working very well. At least from the project management side.

There are IMHO three management angles to look at any engineering project - product, project and architecture. If you are building a house, you need a blueprint to tell where to put what concrete, you need a render (or paper model) that you show to a customer, and you need a BOM and a timeline to make the investors happy. The software is not different. But that's also where there are misunderstandings in what Agile is - the product management, project management and engineering all have different ideas what kind of "plan" is needed.

So in the case of software, specs are like the house's blueprint. In some cases, specs might be useful prototype, in some cases not. It's just not the type of plan that the project or product management cares about.

Regarding the project management angle, for me Agile today is clearly Kanban, and almost everything else is wrong or not required. I often make an analogy with computers. In the 50s and 60s, people tried to plan the work that the computer calculates by creating some scheduling algorithms that plan ahead the use of resources, avoid conflicts and such. Eventually, we found out that simple dispatch queues work the best, don't estimate at all how long the task will take. Just give it a priority, a time slice, and let it run. And I think the same applies for the SW development. And I think it's time that project management people take note from computer scientists - they already know.

Doesn't mean SW development time cannot be estimated if you need to, it's just not a very efficient to do so (it takes extra time, depending on how good estimate you want).