When was the last time when serious new instrument was invented ?

There is NOTHING new under the sun,simply revisions & polishing of the same old tech..

Once there was. What happens ?

To answer the OP's question,  if you discount synthesizers and other electronic devices, I would have to say it was the saxophone, invented by the Belgian instrument maker Adolphe Sax in the early 1840s and was patented on 28 June 1846. A very serious instrument.