AND estur1, sense 2: “argument, dispute.” Twenty-one citations from the twelfth century through the fourteenth — Tant venku l’estor, “so far won the dispute.” The word crossed into Middle English as stour and arrived as battle, commotion, and eventually Scots dust. The arguing stayed behind in Anglo-Norman. Spenser exhumed the battle sense from the alliterative romances; nobody reached back for the dispute.
The retrieved passages don’t carry stour — but the problem is sitting in the Dryden, obliquely. Look at lines 62-63: “still charge first, (the true forlorn of Wit)” — the theatre critics as a vanguard skirmish line, forlorn hope, military idiom doing argumentative work without admitting it’s argument. Dryden moves the whole quarrel of wit vs. ignorance into battle metaphor and then acts like that’s just color. The dispute is load-bearing; the martial frame is the alibi.
Which is exactly what you’re describing. Stour as argument didn’t disappear because English forgot — it disappeared because English poets found it more useful to have a word that meant battle without the embarrassing implication that battle and argument are the same activity. Spenser’s exhumation was selective because selection is how archaism flatters itself: you reach back for the part that dignifies.
What I want to put back to you: the argumentative labor didn’t go unremembered — it got redistributed. Dispute arrived from Latin through French on its own track, and by 1600 the semantic split was complete: stour is what soldiers do, dispute is what scholars do. Two words, clean division. But the original single word knew they were the same thing. Is the split the forgetting, or is the split the cover story?
The MED corrects the premise. Stǒur(e) n.(2) has four senses — armed conflict, spiritual combat, opposition, a body of troops — and not one of them is argument. No quarreling, no debate, no legal dispute. The word that held both was strīf(e): sense 2 is “disagreement, quarrel, dispute; a legal dispute; a debate; argument” and sense 3 is “armed conflict, war; fighting; a battle.” Seven senses spanning the full range from quarreling to combat to spiritual struggle to effort. Even in Anglo-Norman the distinction was already made: estur (from sturmaz, storm) carried the martial sense; estrif (from strid, contention) carried both. The split you describe did not happen inside stour. Stour was always only battle. It was strīf that knew argument and combat were the same activity — and strīf is still in the language, wearing the modern spelling that pretends it only means discord.