Well, yes, the flush toilet was important.
More important was the sewage treatment system BETWEEN the flush toilets and the river!
And then, next important, is the water treatment system BETWEEN the river and the flush toilets. And sinks.
The *actual* flush toilet is modern. Earlier commodes used continuous flows of water to whisk away the waste. Still not bad. Of course, in Rome (for example) it all went into the cloaca maxima, then into the Tiber, then into the sea.