Post-Cold War assumptions behind submarine cuts

Was Russia really expected never to become a military threat again after 1991?

Yes; contemporary policymakers broadly expected lasting peace and discounted a renewed Russian threat.

Many policymakers genuinely treated renewed Russian military danger as implausible after 1991. They also expected China and Russia to integrate peacefully into a stable, US-led international order. That optimism made conventional capabilities appear expendable even though relatively modern Russian submarines still existed. These assumptions explain the force reductions without making the assumptions accurate.