Wait, that can't be right, it would be ludicrous. Once a game passes the threshold it pays per-install permanently?
Yes and no. You need to meet _both_ thresholds, cumulative (lifetime) installs _and_ yearly(!) revenue. I (!)'d the yearly part there, because you still need to be pulling in a yearly $1M of revenue (I'm assuming Unity Pro here cause the math is simpler) after your 1M of installs.
So while there are some edge cases here that are legitimately ludicrous, it's not the case that you're on the hook for the game in perpetuity, because if your game falls off a cliff and you make $500k in revenue next year, you owe nothing in runtime fees. In other words, you're not incentivized to take it off the market after 1M installs unless the runtime fees made it so you started losing money on the game after your $1M of revenue-- there are some examples where this is possible but none of them are very realistic.