I think the capex being fixed assumes you can just stop training the next model. But its not clear that you can afford to do that and keep selling tokens.
And if capabilities plateau such that training the next one is useless, then the margins will drop fast due to competition.
Model inference:training compute for frontier models is estimated to be over 10:1 now.
Driven mostly by just how much inference they sell nowadays - but also by things like base model reuse.