> >> 2. (from Suhas) "once a valid transaction is created, it should not
> become invalid later on unless the inputs are double-spent."
> > This doesn't seem like a huge concern to me
>
> I agree that this shouldn't be a concern. In fact, I've asked numerous
> people in numerous places what practical downside there is to transactions
> that become invalid, and I've heard basically radio silence other than one
> off hand remark by satoshi at the dawn of time which didn't seem to me to
> have good reasoning. I haven't seen any downside whatsoever of transactions
> that can become invalid for anyone waiting the standard 6 confirmations -
> the reorg risks only exists for people not waiting for standard
> finalization. So I don't think we should consider that aspect of a
> sponsorship transaction that can only be mined with the transaction it
> sponsors to be a problem unless a specific practical problem case can be
> identified. Even if a significant such case was identified, an easy
> solution would be to simply allow sponsorship transactions to be mined on
> or after the sponsored transaction is mined.
>

The downside is that in a 6 block reorg any transaction that is moved past
its expiration date becomes invalid and all its descendants become invalid
too.

The current consensus threshold for transactions to become invalid is a 100
block reorg, and I see no reason to change this threshold.  I promise to
personally build a wallet that always creates transactions on the verge of
becoming invalid should anyone ever implement a feature that violates this
tx validity principle.