you are of course perfectly right here. I just wanted to suggest a criteria that would be very simple to implement and still capture the main effects. A debt/GDP-ratio would be the perfect solution but I doubt that GDP is calculated in Victoria II. Given the closed money circuit Paradox implemented for Vicy II I'm wondering if it would be possible to calculate GDP and GDP per capita numbers. I guess that should really be something players would appreciate as a performance feedback. It would also provoke alternative economic development strategies. Democracies might want to maximize the consumption power of their citizens (GDP), while autocratic regimes might aim for an industry that is better suited for warfare (heavy industries, arms...). The well known industrial index might be a weighted index of the GDP and several output numbers of strategic industries.
It would be very easy to calculate GDP in Vic. In this game the profits factories and RGOs equal the added value they create (output minus inputs). Thus take sum of all the profits of RGO, then add the sum of all profits and losses of factories and you have a GDP figure.