To risk permanent harm to the system...
There is no risk of permanent harm.
Hmm. I'm going by what I've read on the other thread. I seem to recall one of the big points of contention was that, essentially, "we can't screw this up, because if we do, we won't ever be able to fix it."
One example:
And, at the same time, there's nothing stopping anyone from implementing both. It's conceivable someone could create a client that did the validation for both BIP-16 and BIP-17. Miners could also validate both style transactions. Since they are both designed to be backward compatible, they will all be recognized as legitimate transactions by the whole network if they get into a block (even if not all clients relay them). The downside of course is that if you create either a BIP-16 or BIP-17 style transaction and the majority of mining power isn't doing the full validation, you run the risk of losing your coins to theft.
Perhaps its more accurate to say implementing either risks causing serious problems.
And the blockchain bloat? That almost seems like a distraction. Complex transactions are going to take up extra space in the blockchain; arguing over where and when the bloat occurs seems pretty academic too.
You forget that completed transactions can be pruned. They can and will be deleted by many miners after the bitcoins are spent further. Unspent bitcoins can not. They are kept by all miners forever. These BIPs would reduce the size of unspent transactions, no matter how complex, to the lowest safe size. It is not merely academic. It is significant.
Unspent bitcoins are only kept by miners until they are spent.
Sure, there will be some "lost" coins... coins sent using complex transactions where the recipient loses the private key, dies before spending the coins, etc. But I think that's a negligible situation that shouldn't dictate the protocol.
I think the real question is, how long on average will coins sent via complex transactions remain unspent? If the intention is for most transactions in the future to become complex (which I'm seeing hints of, and if so, I'd love to be pointed to where this is actually discussed openly,) then I see your point, but I don't see that that as a given.
Even if the average time delay of spending coins from complex transactions is months (which sounds highly unlikely,) it still just delays the pruning. A "rolling amount" of temporary bloat from some unspent complex-tx coins doesn't seem like it would be enough to worry about.