In this quotation:
Theseus was a great hero to the people of Athens. When he returned home after a war, the ship that had carried him and his men was so treasured that the townspeople preserved it for years and years, replacing its old, rotten planks with new pieces of wood. The question Plutarch asks philosophers is this: is the repaired ship still the same ship that Theseus had sailed? Removing one plank and replacing it might not make a difference, but can that still be true once all the planks have been replaced? Some philosophers argue that the ship must be the sum of all its parts. But if this is true, then as the ship got pushed around during its journey and lost small pieces, it would already have stopped being the ship of Theseus.
Question: In the last sentence,
it would already have stopped being the ship of Theseus.
Why did the writer use the grammar form "would have [stopped]"?
Is it just the expression of indicating "guess" or a part of "subjunctive mood"?