There's a bit more nuance here. Fron this article, the plan is to treat the water to decontaminate it, then dilute it as much as possible because the treatment cannot remove some isotopes which could cause problems. The 30 year plan is actually a good thing since this would dilute the isotopes further making the risk minimal according to IAEA and the US. There are some independent labs that voice concerns for more data though.
The main issue is that the tanks that are supposed to hold the contaminated cooling seawater are filling up quick, so they need to add some space. Unless there's a better plan, it's either that or the tanks overflow.