let's say you write a search engine, a route planning system, a database, rule-based NLP (for most ML-based approaches fast, native math libs + python may be great enough because nearly all the time is spent in the math part) on HUGE datasets, etc, etc

pretty much any system involving long list- or set-like types with a decent level of complexity. Most other languages enforce far too much indirection, padding et. al for collection types. Especially generics fail miserably compared to templates for that particular matter. C, on the other hand, will make everything MUCH harder. Especially proper use of RAII and (const) references allow (almost) zero overhead and almost no error-prone pointers (especially zero pointer arithmetic) in the whole system.