I've no idea what 'boastful surety' is, but here is the intuition which motivated me, which the reader may find useful:

PoW has a value. The electricity burned by the entire bitcoin network attempting to produce one block costs 25 BTC on average, because overall, PoW mining is equilibrium centered around break even. Therefore, as the recipient of a transaction for 25 BTC, it stands to reason that I can wait for 25 BTC worth of PoW to get appended to the chain after my transaction appears in a block, because any double spend of this transaction after that point would not be profitable for the miner, since they must expend that amount of PoW to achieve the double spend.

In a system with no block reward, I can't see how you can possibly make any judgement about when it is safe to accept a transaction, because there is no direct valuation of the PoW.