They're not mutually exclusive. You can learn a little bit of a lot of things on the path to "going deep" in a single topic. I believe this is how you end up with T-shaped skills: https://en.wikipedia.org/wiki/T-shaped_skills
"T-shaped skills" or whatever you want to call them + the capacity to learn is quite powerful.
- become a senior with one topic (you don't have to be the ultimate master on this) - have mid-level skills on topics surrounding that one main skill - have a junior-level skillset on the outlying topics - have some knowledge of anything beyond that
For example: a back-end developer should be really good at back-end development but they should have a very good understanding of the front-end, DevOps, and DB as well. They should have some rudimentary understanding of how UI/UX works and system administration as well. Beyond that, they should have basic ability to understand what's being discussed regarding product development, marketing, how the OS works on bare-metal, etc. etc.
I feel like that makes a more well-rounded employee that has an easier time moving around their space, understanding their co-workers, and, worst comes to worst, shifting to a new topic when it becomes relevant.
Too much focus and you can't see the forest for the trees. Too much general knowledge and you're a jack of all trades, master of none.
Finally, note that "balance" is subjective. It's the balance that works for you and where you want to go. Deep, focused experts are valued. Generalists are valued. Find your niche.
Going wide also has benefits as you'll stumble across things that have surprising application in the areas where you've gone deep.
Why is the better than being n or m shaped (great at 2 things; fairly good at 3 things) and so on?
It seems the point about the deep skill going obsolete still stands.
Personally I like the octagram !!!