Long-term
Each upgrade made sense. The better apartment because you were both making more. The nicer car because the old one was unreliable. Private school because you wanted the best for the kids. Vacations because you'd earned them. None of it felt like a decision. It felt like a natural progression.
Years later you're doing math you can't make work on one income.
The game
Standard of living is easy to raise and hard to lower. Each upgrade resets the baseline. The private school becomes non-negotiable once the kids are enrolled. The apartment becomes the floor, not the ceiling. The lifestyle you built together requires both incomes to sustain, which means it requires both of you to stay.
The dependency doesn't have to be engineered. It accumulates through ordinary decisions made in good faith. But the effect is structural: leaving becomes more expensive with every upgrade, and the person with fewer outside options is more trapped than they appear.
The equilibrium
Each step was genuinely mutual. Neither person decided to trap the other. The trap assembled itself from reasonable choices that each made sense in isolation.
The ratchet only turns one way. You can upgrade into dependency quietly, over years. Unwinding it requires an explicit conversation about what you can actually afford alone, one most couples never have until they're forced to.
Dominated strategies
Continuing to upgrade the lifestyle without modeling what it looks like on one income is the dominated strategy. Each additional commitment makes the exit more expensive and the dependency deeper.
The better move is to maintain independent financial footing throughout, not as a preparation for leaving, but as a hedge against the situation where you have no choice. Know what your life costs on your own income. That number is the one that matters.