Yes, that’s correct. However, archetypal ECS operates under the principle that entities will not take on all possible combinations. For example, if you have 900 pixies, 2 orcs, and 1 dragon, assuming no status effect, then you’d have 3 archetypes. In the worst case, you will have as many archetypes as the entities. In practice, many entities will share the same archetype. Again, think of warriors fighting on a large battlefield or something along that line. Maybe some of them would be afflicted with status effects, but these are likely highly correlated.
Flecs is an archetypal ECS in C. It stores entities in archetypes. It doesn’t actually compile fused updates like my plan. (Though a manual fusion benchmark suggests that if possible, fusing updates would actually increase performance) Initially, I had the same apprehension as you do regarding combinatorial explosion, but in practice, Flecs works.