Restaked ETH does not secure Ethereum, generally speaking.
For any given amount of stake to be actually securing Ethereum, the actor that is taking on the capital risk of staking failure/chain split MUST be the same actor that is making decisions about what algorithm (client) to run. As soon as you either outsource algorithm choice to someone else or you sell the risk to someone else the game theory for staking falls apart and your “stake” needs to be treated as attacker controlled stake for mechanism design calculations.
The above game theory issue is the reason why solo staking is so important. There is no such thing as “too big to fail”. We can wipe out 99% of stake and everything will be fine as long as the wiped out stake is of people who chose the wrong algorithm (e.g., chose a super-majority client, or a censoring client). There will be a long period (weeks) of no finality during recovery, but the system is designed to self heal from nearly all staked ETH defecting.