So if you envision doing programming like that outside HPC after your PhD, C++ might be worth the investment.
But of course a lot comes down to what language other people in your group and your particular subfield, and what your supervisors are using. Doing a PhD is hard enough without being the odd guy out who's using a language nobody else in your community is using.
Some older folks used Perl in places where others would've just written bash; mostly scripting invocations of actual number-crunching programs. Ruby would've raised some eyebrows for sure!
I chose Python over perl about 25 years ago and never would have considered ruby or php (both seemed squarely aimed at web developers not scientists). I believe Numeric Python was released around that time, and it was a revelation- especially to matlab users, who recognized the syntax and behavior.
Python has a low barrier for entry and can be useful immediately to newer programmers. It is also fairly well rounded.
That being said, I am not partial to python as OP was with fortran, and steer around it regularly.
What? Why???
I switched from Python to C++ because Cython, Numba, etc. just weren't cutting it for my CPU-intensive research needs (program synthesis), and I've never looked back.
The kind of C++ written for HPC tends to be much simpler than the kind of C++ used to write e.g. database engines. The complexity of C++ is not that onerous in context and Fortran isn't exactly an exemplar of ease of use.
In my effort to try to use Python instead in my field, I fell down countless rabbit holes along the way (like being for a year or so one of the core contributors to Cython, in an effort to make "a better Fortran").
In the end though -- I decided it was time to just Get It Done, and just did what my supervisor had patiently and subtly hinted for years. Enough rabbit holes, just get it done in Fortran, and hand in the thesis...
I think what I learned most about (having an interest in compilers and programming languages) is the ignorance the rest of the programming language community, and computer science, has about the kind of things you are actually interested in when doing HPC. What kind of language features you care about and what idioms make sense.
Learning Fortran is super simple.. that is perhaps part of the problem that also people who are barely programmers can write (bad) programs in it. So what this must mean is simply a lack of programmers, full stop, being attracted to the field.
And...C++ is truly the most wrong language for this space there could be.