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

It's already happening. There are AI music generators like Soundraw, Loudly and Suno. All AI requires is a frequency and timbre generator. Timbre can be sampled from any existing sound. Composition will take a little longer. At first it will all sound similar, much like today's pop music. Being creative and composing different-sounding songs and genres will be more of a challenge because AI only uses existing data.

The Incredible String Band used anything they found lying around!

More on the AI front. I doubt if a new instrument will ever be invented. I've been going down the AI rabbit hole for a while, mostly for investment purposes but as a former software engineer I've gotten obsessed with the technology also, and I'm fully convinced music as a commercial endeavor for humans is dead. Apps like Udio and Suno are already capable of generating high-quality songs. Rick Beato, a former music professor, producer and guitarist with over 4 million YouTube subscribers says an AI-generated song will top the charts within 2-3 years and within 10 years 9 of the top 10 songs will be AI-generated. After what I've researched, I totally agree. So that leaves hobbyists to invent new instruments because there will be no money in it for businesses to do it. The only reason to play an instrument will be for a sense of accomplishment.

The more I learn about AI, the scarier and at the same time, amazing it becomes. Many "generative" jobs will disappear much quicker than people will imagine, like musicians, artists, writers and authors.