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It's definitely not *obvious* that final supply is independent of starting amount, but it makes sense on reflection. If you have a continuous source and sink (imagine a container of water with a inflow and outflow), the end state (level of water) will obviously not depend on the starting amount of water. For the rest, sure, agreed.
One small nitpick with the analogy, not to be annoying, is that the inflow is constant size while the outflow is proportional to total amount. More like a water balloon with a hole that stretches as you fill up more, and a constant hose filling it up