This bookcase doesn't look that sturdy, so let's buy another one.
This bookcase is kind of flimsy, so we should think about getting another one.
この場合、次のような言い方ができると思います。
ーThis bookcase doesn't look that sturdy, so let's buy another one.
「この本棚はあまり頑丈じゃなさそうだから、別のを買おう」
壊れそうなことは、doesn't look that sturdy と表現できると思います。
ーThis bookcase is kind of flimsy, so we should think about getting another one.
「この本棚はちょっとちゃっちいから、別のにしようよ」
flimsy「もろい・壊れやすい」
ご参考まで！
カナダ 
