It is far from a philosophical argument. It is a significant economic argument. You chose not to purchase a single ticket for what I presume is a good reason.

But, you want the benefits of the single ticket. Generally the baggage trick works. But, not always.

Sorry, on separate tickets, you get the convenience of not physically moving your bag around, perhaps missing onwards flights, and in inconvenient immigration situations, not having to deal with a visa (or at worst, not being able to retrieve your bag at all). But, you don't beat the baggage fees.

From an economic perspective, you say that this is a good thing. Perhaps for you. But, for SU which wound up having to deal with a bag misconnect, those extra fees mattered. More importantly, as the last delivering carrier, had the bag been lost or destroyed, it would have been SU which would have coughed up the Montreal Convention payment to you.

But, it doesn't really matter. Fact is that you are correct that the usual practice is that onwards carriers don't bother because these things even out over time. But, it is a good point that SU can't be relied on for this.