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

The cowbell is an idiophone hand percussion instrument used in various styles of music, such as Latin and rock. It is named after the similar bell used by herdsmen to keep track of the whereabouts of cows. The instrument initially and traditionally has been metallic; however, contemporarily, some variants are made of synthetic materials.

All kinds of unique and interesting Eurorack modules coming out weekly.

Every new pipe organ is unique. 

Since electric guitar is two-component instrument, the Kemper profiler made guitar amps obsolete about 10 years ago. Other digital devices, by Neural and Fractal have expanded that transition. In live music almost no one uses a "real" amp anymore. In blind tests no one can tell the difference, although as in audiophilia there are some who stubbornly hold on to the belief they can despite their own failure in blind tests, but they will age out because almost all younger players have gone digital. I now play strictly through software and have sold my "real" amps.

If AI can be considered an instrument it is the newest, although it mimicks current instruments. But it will eventually replace musicians. Especially those who play instruments that can't shift pitch easily.

AI will replace musicians only if it also replaces us. In all other scenarios it will not.

However, this real computer revolution is coming, what we have now is child toys, the very beginning. I don't want to even think about it.