Does CPI always go up? It certainly doesn't have to.
It doesn't actually have to, but the experience of history is that it almost always does. Examples of deflation are rare, while inflation caused by increased money supply certainly goes back at least to the Romans, who cleverly started mixing copper & other base metals into their gold coins.
I think the motive should be obvious: whoever controls the money supply gets free money. Like counterfeiting, except you don't have to do a lot of hard work engraving plates & setting up your own printing plant :-)
But that's why personal CPI is better. My personal CPI can go down if, for example, I refinance my mortgage at a lower rate and that saving outweighs the increase in corn prices due to bad weather (or whatever).
That's why I think it's useful to distinguish between a CPI and COL (Cost Of Living). If gas goes up, my personal CPI goes up, but if I replace the guzzler with a hybrid, my personal COL index goes down.