I find data.table code as obscure as base R if not more so. Purported brevity (always illustrated only on very simple problems) doesn't compensate me enough for the obscurity.
I could get used to it, I'm sure. I could also get used to doing everything in base R, pandas, or Clojure. Doesn't mean it's a good use of my time.
I get the sense from how some people blog that basic "filter + group by + summarize" type pipelines on giant datasets are all they really do. data.table may be a great choice for that kind of thing. But I use dplyr as a tool of thought. I can solve complex, novel data processing and analytics problems in dplyr more precisely and efficiently than I can explain in words. Instead of having an idea and then having to think about how to code it, the ideas translate into dplyr in an obvious and automatic way.
All that said, data.table's authors have worked much harder to provide advanced functionality, most notably non-equijoins, that it looks like the tidyverse may never catch up to. So if you need that kind of thing a lot, you need data.table or maybe DuckDB.