How about fullstack developer using entirely different technologies on the server and client side. Just be an expert in everything and do it all.
It’s not that odd to be good at two languages.
Back in the day we had client server architectures and it was exactly the same thing. Even more tiers would sometimes be added and there were frameworks for all of them. We didn't get source code. The frameworks were full of bugs. There was no stack overflow. Documentation usually consisted of a few trivial examples. You just had to hack, hack, hack and figure out how it worked. I remember when Swing came out and being very excited about being able to work on something that wasn't completely insane for client stuff (that's how bad it was ;-) ).
This is just the life of the computer programmer. If you like learning new things, it's never a dull moment!
I think a lot of the worry comes from developers that are relatively inexperienced. We put "senior" tags on positions filled with people with only a few years of experience. Sometimes I think it's hard for newer developers to imagine what it's going to be like 10 or 20 years down the line. Being good at 2 language (or frameworks or whatever) is almost laughable. If you don't have half a dozen hanging from your belt once you've got a quarter or a third the way through your career, I think it's a big problem that you should aim to fix. Picking up something new and being effective and efficient with it very quickly is an incredibly important skill -- one which no programmer that wants to work in the field long term should avoid.
Now, if you happen to have a very young team that realistically can't handle the challenge, then that's a completely different story. It's also very important to understand what your team is capable of.
I find knowing both backend and frontend to be hugely important in the design of both systems independently.
I've been involved in tech for 2 decades now. I currently follow frontend / backend JS, Docker/Kubernetes, native mobile as well as RN and Flutter, and general DevOps. In the past I've done Office add-ons in C#, game bots in Java, PHP pre 3.0, Visual Basic GUI wayyy back in the day, done API's in Python/Ruby, and used languages I don't even remember anymore due to preexisting codebases or the team choosing something that fits better.
I've been a full stack developer since my very first job creating a Java based web app for a startup where the only other coder was the CIO. The CIO didn't know Java and primarily worked on C/C++ embedded code for unrelated projects. Back at this time raw Java servlets were still "the thing" and javascript with jQuery was state of the art on the front end. With just these primitive tools I built a fairly modern even by today's standards reactive web app. Lots of stuff loaded dynamically based on user interaction without switching pages. It was a legit web app, not just a web page. Actually, I was more than a full stack developer. I was also the sysadmin, dba, and network engineer. I physically installed the hardware at the AT&T data center, created a DMZ with the firewalls, configured load balancers, installed the DB and set up a hot backup, literally everything that needs to get done to make a working web application. There was no one around to learn from, it was all web searches: how to secure Apache, how to set up certificates, how to create indexes on the database... everything.
I ultimately decided I prefer back-end development so that's where I spend most of my time these days. However, in recent years I've picked up Angular and Backbone because sometimes the front end team needs some extra bandwidth and being able to help out is really valuable. I'm not as good at that stuff as the dedicated front end guys. I certainly didn't do as good of a job setting up the network or eeking out max performance on the database at my first job as dedicated professionals would have. But I got the job done at a professional quality level. My first job was surely a rather unique experience, but everywhere I've worked multiple people on the team have been comfortable all the way up and down the coding stack, even if they might have specialized in just one area.
That sounds completely normal.
Typescript/JavaScript front end.
Python, Java, .net on the back end.
Database written in C++
OS written in C.
There are certainly jack of all trades developers that can do that for you. You will get a jack of all trades solutions that may have oddities here and there. Which could be okay depending.
Naturally we are doing it full stack.
In the same vein, developing native apps with three tiers, while using stored procedures for most of the business logic.
Naturally also knowing about the technologies across all tiers.
Compare implementing back-end form validation in PHP and a completely different implementation front-end validation in JS vs using the exact same implementation on both sides. The former approach requires more work, creates a larger maintenance burden and a greater area for inconsistency and error because you need to maintain functional consistency between the two different implementations in the two different languages