> Seaweed grows very quickly
This is like saying that trees grow quickly.
"Seaweed" means thousands of different species. When we see a cluster of seaweeds in a rock some of them are not even in the same kingdom than their neighbors. Its life cycles and chemical properties are totally different.
The truth is that some species are really picky and some take decades to regrow after harvested. Just because brown seaweeds are fast growers does not mean that red seaweeds are.
> there's been relatively little need for seaweed in recent decades
You may not be aware of how much of our everyday stuff includes algae. The demand of toothpaste has not fell. I would dare to speculate that the demand of laboratory stuff is probably higher than ever since Covid.