[zozbot123]

> the cost of decrementing/incrementing these counters is very real

Only true if you use pervasive reference counting, with atomic updates. Swift essentially does this, but other languages don't.

One of the reasons why it makes sense to single out tracing GC (as opposed to RC) is that it's essentially a package deal; your whole program needs to be structured to use memory as essentially an in-RAM database, where every single object or reference can be traced after the fact at some arbitrary time in the future. (There are reasons to do this in some cases, even in 'low-level' languages which are not ordinarily garbage collected - see e.g. the use of "entity component systems" in such languages - but it's obviously not a generally-appropriate solution in any real sense!). RC is self-contained, and it need not even be locally inefficient.

> You're talking about gradually typed languages.

Interestingly, gradual typing does seem to end up with a "worst of both worlds"-type situation - the dynamic tag checks and conversions that have to be inserted at every boundary between statically- and dynamically-typed parts of the program introduce pervasive inefficiency, which is only resolved when static typing is used essentially throughout.

[ncmncm]

The term for languages like Swift, Java, and Haskell is "obligate GC". The cost of obligate GC is only superficially just the poor memory locality, interruptions, and bad citizenship -- worse is that the power of destructors is denied you, and with it the power to automate managing every other kind of resource.

[lmm]

> worse is that the power of destructors is denied you, and with it the power to automate managing every other kind of resource.

These are orthogonal concerns. Plenty of languages without GC do not have good support for ARM (e.g. C), and a GCed language can still have good support for ARM (e.g. monadic regions in Haskell). C++ is really the only language that conflates the two, and IME its RAII style is overrated in practice (there are a lot of rules you have to follow to avoid leaking, and for many of them you get very little warning if you break it).