I disagree with all this part. The way things are going, being a jack of all trades will only make you irrelevant very quickly. The market needs experts, and being an expert means focusing on one thing and one thing only. You should only learn new tools if they're relevant in your field of expertise and if they're widespread or there's a good chance they might be in the future. Otherwise, you're just wasting time.
Learning how to learn is also a valuable (meta)skill. It doesn't mean you have to become a jack of all trades but does allow you the opportunity to reinvent yourself if the need or desire arises.
I think there should be a middle ground there. You can know a lot about code optimization but if you don't have an OK understanding of the lower parts, you will spend lots of hours discussing with sysadmins or netadmins about how it is their or your fault that you application is not behaving harmoniously with the OS, for example.
Being a jack of all trades is not a bad thing most of the times. It helps give you a better understanding in the end.
Breath and depth everytime. Being am expert can open the door and give your words weight, but you need to be able to talk to, relate to, and understand people in other fields. You also need to be able to adapt to change and to borrow concepts from other fields.
What the market wants ("Swift developers with ten years' experience"), what we as labor want ("Fun projects and a paycheck for whatever we make"), and what's actually required ("Wordpress ecommerce theme that's mobile friendly and doesn't look like complete ass") are all different.
The specific demand is invariably for a particular technology or platform, because management wants to solve their pain point right now and want to manage risk. The problem is that those experts are not super useful in five years when the platform changes (or three years, or six months).
Better to be an expert as using different technologies and getting up to speed quickly.
Yes, I still see some job postings that list "8+ years with (niche technology or framework)" (I say niche because asking for 8+ years with something like Java or Ruby isn't exclusionary to generalists, it just means you've predominantly focused on a single ecosystem), but far more often I see "X preferred" "Some exposure to X, Y, Z, or similar preferred", etc. You still want to have deep understanding of a few key technologies, sure, but there's risk hitching yourself too closely to a given tech while excluding everything else that's out there.
A wise man in my company asks younger ones to follow shape of "T" when it comes to skills. Get as much breadth as you can but do ensure that you are deep in one area.
I disagree with all this part. The way things are going, being a jack of all trades will only make you irrelevant very quickly. The market needs experts, and being an expert means focusing on one thing and one thing only. You should only learn new tools if they're relevant in your field of expertise and if they're widespread or there's a good chance they might be in the future. Otherwise, you're just wasting time.