Interesting. But I would have to constantly thinking whether to keep or let my note expire every time I post it. Besides what will happen to the replies? If one reply in a thread of reply expires then the whole thread is meaningless, ie if other people don't let their replies expire.
Discussion
You don't have to constantly rethink it, as I wrote you can create some software UI on your end that automates this. Like a rules engine. Example: Automatically re-post all notes with a certain hashtag until the rules gets manually canceled. However that's something that's strictly on your (sending) client end and don't need to concern the relays or receiving clients.
As for replies in threads, the sending (posting) client sets the expiration date but it's up to the relays and receiving clients to interpret it. It's more like a hint than a mandate. We can't mandate deletion anyways we've had that discussion. So a relay could keep expired notes around if they're required for thread integrity, but receiving clients should highlight the fact they're expired e.g. by graying out.
We need to arrive at some set of conventions across the ecosystem.
Fair. This might lead to a significant reduction in costs.