Work is not lost using p2pool, those stale p2pool shares can still solve a block and still be submitted to the bitcoin network for reward. The time and work invested in them is not lost. Those shares just don't show up in the p2pool sharechain, but will show up in the bitcoin blockchain.
Doesn't a winning (bitcoin) block not override the share chain? Otherwise, that block couldn't actually be paid (by the share-chain)
Well, it depends on what you mean by "override."
A winning bitcoin block is sent to the bitcoin network and to the p2pool network. Bitcoin will payout the reward normally for the found block (including the finder's share). However, if the winning block isn't on the longest chain of the p2pool sharechain, it will get orphaned by the p2pool network.
In that case, the winning block is paid out and the solver will get credit for it for the current block (in addition to the 0.5% bonus for finding it), but the person that solved it won't get a credit for the share it for future blocks on the p2pool chain. This is fair because everyone has the same chance of it happening, and it does not mathematically affect expected payout for anyone. Additionally, this is extremely rare (<0.1% of blocks found will have this).