It make a a bit more sense when you think about it - Risk isn’t really the main thing incentivized here, making interest payments is haha. Risky lenders don’t make payments, but neither do people who never take on debt

Credit scores actually are designed to measure risk, not interest payments. Otherwise high credit utilization would be a positive:) There are certainly edge cases where the score has low predictive value, but overall the correlation between credit score and risk is quite large.

The closed card drops off your account history after ~10 years, so your score shouldn't immediately go down. Technically, I'm not sure that the credit scores can differentiate between you closing a card and the bank closing your card (the latter might be some sort of signal of potential change in creditworthiness)

In general, credit scores are optimized for the average consumer - and so behaviors around the edges don't always make sense in terms of your score going up or down. If you think about the system that was replaced by credit cards (i.e. keeping a tab open with many different stores), you'd be hard pressed to say that the amount of credit extended is lower now (overall), than under the old system. Whether that's a good thing or not is a different question.

In general, your score would immediately go down as your credit utilization would go up (outstanding balance over total available credit).

0/infinity is still zero

…depending on the languages used in their backend and the quality of their development efforts…

The balances are instantaneous balances, not ones that are being financed. So, if you use and pay off your card every month (as makes sense to me for points/cash back, ease of chargeback, and convenience), then your numerator won't be zero on most days. (For me, it's not $0 on any days, because of the timing of statements.)

You could theoretically pay off the card before the statement posts to keep the balance at zero. Usually not worth it, but if you want to boost your credit score before getting a mortgage, that trick usually works.

In Canada, Equifax et al tend to reduce score at each over 80% revolving credit account utilization.