"No, it's less effort than that"

If developers did not approach projects with the goal of adding acronyms to their resumes and infuse every project with the latest cargo cult du jour it would improve the ability to predict timelines.

Pick boring technology, that the team is already comfortable with, when possible. Keep the teams as similar as possible, Keep running projects the same way, when possible

I am not saying it will get things perfect, but it will be an improvement.

Developers do not choose the tools. Not in agile/scrum companies. Heck, they are not even allowed to choose the variable names by themselves.

What? That seems like it would vary from company to company. My company does scrum and developers choose which technologies to use, which seems like it's probably common...

The problem is that developers have a very strong incentive to add new technology to their resume, as their job is not guaranteed. I think most junior devs have seen some old hat at their first real job who codes in one, ancient tech stack; someone that they perceive as un-hirable if the economy takes a turn.

We all see the job listings out there and the tech they desire. It is self-preservation, and it is in direct conflict with what is best for the company itself. To use myself as an example: I have zero regrets with pushing for JavaScript over VBScript many years back, for moving from Microsoft HTA to React, or for moving from Yahoo's YUI to jQuery. Being infused with old tech can very severely limit your ability to survive. Most of my interviews after my first job were simply explaining/defending the ancient tech I had specialized in. It is all risk, no reward at the employee level.