The Geekbench 5 approach of pretending Amdahl's Law doesn't exist is sometimes a valid benchmarking strategy, but generally is the wrong choice for benchmarking consumer workloads and devices, and that's what Geekbench is ostensibly targeting.
The fact that Geekbench 6 scores don't increase linearly with the addition of more CPU cores is not a weakness of the benchmark, it's the benchmark demonstrating an important real-world effect.
The change that Geekbench 7 makes to exclude some subtests from the multicore suite entirely will definitely have the effect of making the overall multicore score scale better with the addition of more cores, but most of the audience for those scores is going to miss out on the fact that the multicore test now measures a narrower range of tasks than the single-core test suite.
> In Geekbench 7, a workload only runs in multi-threaded mode if the task it models actually runs multi-threaded in real applications. For example, the HTML5 Browser test isn’t included in the multi-threaded suite because web browsers are single-threaded (or lightly threaded).
(One could just as easily argue that the single-core test suite ought to exclude any task typically done with multiple threads in real-world usage.)
But, of all of the 2-pick-only options, choosing a custom mix and calling it multi-core requires guessing your actual workload on both the content of the mixed test suite as well as the scaling profile of the CPU to reason with. On the other hand, just testing "single core" and "all core" at least only requires you to guess based on what you think the scaling profile of the CPU is.
Nothing beats just testing your actual workload, but that doesn't mean all other ways of testing have to be equally good.
Yes, that's pretty much the whole point of Geekbench: to offer non-experts a way to measure CPU performance and produce scores that are relevant to a specific class of devices and users and use cases, rather than artificially inflated.
Benchmarking experts can already use something like SPEC, which does offer single-thread and both types of multi-thread test modes (SPECrate and SPECspeed).