Take the House of Commons, which is the lower house of parliament in the UK. It is made of constituent members (whom some liken to planks...), which are routine

Take the House of Commons, which is the lower house of parliament in the UK. It is made of constituent members (whom some liken to planks...), which are routinely replaced. It remains the lower house of parliament, despite switching out all components on a routine basis. If you collected the ex-members of the House of Commons and made an identical grouping of people out of them, you would not have a second House of Commons - you'd just have a gathering of people. Abstract concepts don't nail themselves to physical objects. Besides, create an entirely new ship out of the discarded planks, and the question becomes not "is that Theseus's ship?", but "why is Theseus removing perfectly functional planks from his ship and replacing them with identical ones?". :)