I disagree here, it is much more nuanced and 10% ETH being re-staked does not mean Ethereum is 10% less secure. You may have principal-agent relationships (in fact you will have such relationships) between operators and delegators, and some of the capital at stake may also be burdened with other conditions, all of this is important to consider the security of Ethereum, not only the worst-case pessimistic view.
Something that could happen is a large EigenLayer slashing event, in which case the first-order consequence is a reduction of the “dollar-amount” security of Ethereum (as long as EigenLayer slashings are properly surfaced to the protocol, which is the point of EIP-7002 for instance). Since lots of validators are exited, the consensus-offered yield will decrease, inciting more validators to join the staking set and recovering the equilibrium staking ratio.
A bit of weirdness may happen during the transition, but is in my opinion mitigated by the following. Let’s say an adversary wants to benefit from the temporary lower amount of stake after a large EigenLayer slashing event. There can be two cases. If the adversary is the one to trigger the large slashing event, then its own stake will be penalised and exited from Ethereum, largely preventing them from pursuing an attack on Ethereum (e.g., a safety fault). If the adversary is not the one, then it cannot really predict in advance that a large slashing event will happen, it needs to command a large enough amount of (non-EigenLayer-encumbered) stake to launch the attack as soon as the slashing event happens. At the end of the day, the bounds provided to us by the theory of consensus are binding: If we have an adversary with 1/3+ of the total active stake, a safety fault of FFG is possible in theory.