While it is a possibility, it is also very remote after a natural disaster someone will pickup those seed phrase randomly and claim the money which is stored in a hardware wallet, for example.
Even though the probability is low, this can't be discounted. Sometimes life events are very surprising and an seed phrase lost during a natural disaster may accidentally end up not under a pile of garbage, rubble, dirt or ash, but close to the surface and catch someone's eye.
In the case of flooding, it would also good to remember that seeds engraved on metallic plates would sink directly to the bottom of the river and miss forever, rather than being easily retrieved by some random person (not even mentioning the amount of rubble and junk there would be after a flood).
Nothing disappears forever. This seed phrase, even under a layer of water and a pile of garbage, can eventually be found by other people, just like dinosaur remains or other archaeological finds are now found. But by that time, you will not care much.
Moreover, having lost the seed phrase during a natural disaster, at that moment you will no longer be interested in whether someone will find this seed phrase, because you have lost it. Forever.
Therefore, backups will never be superfluous.
In the case the metallic plate gets lost and we get through the natural disaster it is also a good option to memorize the seed phrase as an emergency way of recovery. Depending whom you ask, it would make the whole situation better or worse.
Or better yet, get a tattoo with the seed phrase.
You shouldn't try to remember it, after all, it's better to use other methods of recording the seed phrase on hard media, and not on something as fragile as human memory.