How to Stay Relevant as a Web Developer

So what you're saying is, to stay relevant, I have to learn PHP and ASP.net? I'm not sure that qualifies as polyglot programming.

I would think of polyglot as someone who can be effective in a systems language, an interpreted language, a DOM language, and probably shell. I.e. something like C, Python, Javascript, or maybe C++/Java, Perl, Postscript, although that's a pretty wacky combo.

Instead , I would prefer to learn something about "hard" computer science or something from a related non-CS discipline such as Math, Statistics , Psychology and find some way to apply that knowledge.

I disagree with this. It could only be true for a very small subset of web developers (freelancers who are working in a variety of languages for many clients), and even then it smacks more of post-hoc justifications for activity like reading blogs and doing 'research' rather than concrete need. What advantages does reading industry blogs each day really give you? What disadvantages does it have (less time for real work)? There is a strange confusion here between work and play, and the silly illustrations accompanying the post don't help its credibility. Given the hyperbole and excited ignorance that passes for insight on most blogs, I'd say it has negative value to read them every day. Once a month would easily work if you want to keep up to date with current trends, but learning a new language each month as a requirement to stay employed is absurd.

There are many kinds of web developer, but just to take a few examples of those who have no need to heavily invest in learning new languages every month:

The majority of web developers work in-house at a firm where the technology in use is mandated, and only changes slowly (and rightly so), because the business depends on it not breaking. They are not threatened at all by the rise of new technologies, and certainly not on the timescale of months.

Then there are many small web developers who make a good living installing, customising, and maintaining CMS systems like wordpress for a wide variety of clients. They don't need to change technologies every month, and frankly it's the last thing their customers want - these developers have chosen a specialisation and will probably stick with it for decades before changing.

Finally there are freelance developers who work on lots of different projects for lots of different clients. They have to stay a little bit more up to date, but the rate of change is measured in years, not months, and there is plenty of time (years) to shift to another platform if the one you have specialised in is not growing but shrinking in usage. The above statement is incorrect even for the developers to whom he expects it to apply.

Now if the timescale were years or decades, I'd agree that a web developer has to be willing to adapt and completely change the tools they are using, but this is hardly a surprise in a fast moving industry. The industry does not change much on the timescale of months though.