$100 today is worth less than $10 fifty years ago, and accelerating in that direction. We need to go the _other_ way, we should print $500 and $1000 notes. And while we’re at, stop minting cents, nickels, & dimes.
But coins for $1, $2 and $5 is likely more convenient. In particular, coins last like 20+ years in practice while paper degrades in 1/10th the time
I still like $2 bills though
It is very hard to figure out how to adjust the CPI over time; some things in the basket decline in volume (more efficient cars need less fuel per mile driven, but then again have people also changed their driving patterns or not?) and of course products switch in and out (who buys ring binders any more but of course a smart phone is a pretty necessary purchase yet didn't even exist 20 years ago).
Also you want to record the influence of high frequency signals (volatile commodities) yet damp it so it doesn't cause noise (fluctuations) in the signal.
Any then there are wholesale service changes (people eat out a lot more; when I was a kid I hardly knew anyone who could afford to eat in a restaurant more than once or twice a year).
There's a similar problem with GDP but GDP is so stupid that I don't care about that. It was "designed" (actually just spontaneously thought up by Kusnets) as an interim way to get some idea of what the hell was going on in the economy using just pencil and paper until somehting "real" could be developed.