When we open a lightning channel using eltoo / LN symmetry (standard 2/2 multisig), the setup/funding transaction must be recorded on-chain in order to prevent a "double spend" of the funding inputs (and effectively stealing the channel balance).
However, what I do not understand is why that UTXO, now recorded on-chain, cannot be spent via the settlement transaction after the relative timelock has expired. This timelock would otherwise, the way I (miss-)understand it, define the ultimate lifetime of the channel which would be very inconvenient.
What am I missing here?
Including the (great) graphic from Richard Myers to illustrate what I mean: