To get better developers need to increase their area (Breadth * Depth). Going deeper and deeper in one area may make you an expert on that topic, but it is easy to become irrelevant if you have no breadth and a disruption occurs. Of course being wide but shallow can be equally troubling, since you can't do anything that requires more than the basics.
It's more like going "breadth first, depth later" vs "depth first, breadth later". RH argues for the second.
I 100% with OP since if you start "breadth first" you can just exhaust yourself before you've made any significant progress on the depth dimension (because there's so much variety in technologies, a lifetime is not enough to learn them all, and one you try to "spread out" you never know when to stop"). Also, going "depth first", you get clear feedback about when you've went deep enough and need to branch out a bit: things just start feeling too hard!
Otherwise you can end up "repeating your first year for a decade" and end up with a much smaller total area.
>but it is easy to become irrelevant if you have no breadth and a disruption occurs.
Disruption does not occur that quickly that proven-capable humans suddenly become utterly useless. If a major tech company with a good reputation starts blogging about it, maybe read a few articles on it and see if it's interesting enough to spend some time on.
There's a neat bias in development where a disruption implies that people above the developers will 100% use the new disruption tech perfectly, and somehow, only developers are the ones at risk of being left behind. Change usually goes much slower, and usually there is an experimental phase before suddenly betting your entire production infrastructure on new X technology. You can also safely bet there will be tutorials and videos to quickly bring you up to speed if the technology is that disruptive.
Also, you have to remember that technologies come quickly but they can also leave quickly as well, making your time spent quickly adjusting to them worthless. Ask Silverlight developers.
To get better developers need to increase their area (Breadth * Depth). Going deeper and deeper in one area may make you an expert on that topic, but it is easy to become irrelevant if you have no breadth and a disruption occurs. Of course being wide but shallow can be equally troubling, since you can't do anything that requires more than the basics.