A simple burst of memmap + soft fault with 100 or 1000 threads on a normal laptop would tell you that thread contention is real and cache locality gets destroyed. Couple that with pinned threads. You can see the latency increase by increasing thread count. Add to that the motherboard interconnect tax for numa systems. Work stealing is not the way for increasingly many workloads on modern hardware.
Recently we built Dip, our in-house ephemeral + parallel database, and we went with may coroutines + work pinning to the same thread which also nicely becomes numa aware via architecture.
Increasing threads beyond system's hardware cores/threads resulted only in marginal gains of a couple of milliseconds worth of differences on huge workload with large increase in memory (thread stack) used by the massive number of threads.