It is being played like a winner-takes-it-all right now (it may or may not be such a market). So it is a game of being the one that is left standing, once the others fall off. In this kind of game, speeding more is done as a strategy to increase the chances of other competitors running out of cash or otherwise hitting a wall. Sustainability is the opposite of the goal being pursued... Whether one reaches "AGI" is not considered important either, as long as one can starve out most competitors.

And for the newcomers, the scale needs to be bigger than what the incumbents (Google and Microsoft) have as discretionary spending - which is at least a few billion per year. Because at that rate, those companies can sustain it forever and would be default winners. So I think yearly expenditure is going to be 20B year++