Where are you getting that number? There are many different types of supercapacitors, which cary in lifespan wlquite a bit and the conditions (mostly voltage and temp) also have a big impact. The article doesn’t specify the type of cap used.

For instance this paper tests a supercapacitor rated for 2.7V at 2.5 and 3v. At 2.5v the cap is estimated to have a lofespan of 100yrs, with 3v scenario it’s 10.

Keyboards don’t get very hot, and the voltage draw is very predictable so I don’t think designing a long lasting capacitor for this usecase is particularly tricky.

I would be worried about the lifespan of the solar panel.