I suspect that the cost/benefit isn't there. Those who need the "biggest Ultra" will be happy with the previous generation or so, and so they'll refresh that on a 2 or 3 year cycle.
Given that generation gains are not sufficient to make a Max twice as fast as the previous-gen Ultra, a longer cycle is rational. The M3 Ultra is still the fastest M-series system.
Given that generation gains are not sufficient to make a Max twice as fast as the previous-gen Ultra, a longer cycle is rational. The M3 Ultra is still the fastest M-series system.