The trouble with this is that there is little generalization in the utility of these baked-in reasoning chains from one domain to the next, so in the end this is not dissimilar to the CYC project's decades long attempt to encode all of human knowledge into a giant expert system... the hope is that if you make your collection of jagged narrow intelligences sufficiently large then it will look more like general intelligence, not a bed of nails.
I would assume that the gains from this type of test-time compute (and synthetic RLVR dataset) scaling will level out just the same as gains from human training set scaling eventually levelled out, and basically for the same reason - because you are tapping into a finite data pool, whether language itself, or reasoning steps isolated from that language, so at some point the incremental gains become increasingly small (10->20% is a doubling, 90->95% is just a ~5% gain).
It's not clear where all the different AI companies are currently focusing - on some of these narrow verticals, or on growing the forest of narrow intelligences. OpenAI's chief scientist, Jakub Pachocki, said that their current focus is on RSI(!) - improving the model in ways that will help them iterate faster in order to have a "fire meets fire" tool than can combat enemy AIs. It's not clear what this really means - what skill set makes an LLM more helpful in the process of building LLMs, but it seems to basically be process automation.