I think when Agile is finally replaced by something else it will be because of:
Individuals and interactions over processes and tools
Working software over comprehensive documentation
Nothing in Agile has actually overcome the problems of horizontally scaling a team, and things like this stand in the way of scaling one vertically.
Scaling developers vertically requires having support tools, processes, and people to get past the things that trip them up and back to forward progress. They don't have to be big tools, or big processes. In fact it's much more important that they be reliable than that they have huge feature sets.
But Agile without tools and processes? Show me anyone who is pulling that off, and I'll show you someone who has mislabeled a bunch of things as 'not-tool' and 'not-process'.
The difference is one of intent. Having tools or processes to have processes is a bureaucratic mess which they Agile folks rightfully wanted to cut with a scythe. Sword. Scary sharp thing of your choice.
There's a difference between building tools and process, and building a culture of toolsmiths and process tweakers. The latter, I think, follows the spirit of Agile but not the letter of it. And lean is just... throw the baby out with the bathwater. It's bureaucracy as written by a minimalist.
Scaling developers vertically requires having support tools, processes, and people to get past the things that trip them up and back to forward progress. They don't have to be big tools, or big processes. In fact it's much more important that they be reliable than that they have huge feature sets.
But Agile without tools and processes? Show me anyone who is pulling that off, and I'll show you someone who has mislabeled a bunch of things as 'not-tool' and 'not-process'.