You forgot to account for two things.
1) You don't need to store it for 20 years. Since transactions can be pruned, people don't need to store the entire block chain (on clients that support pruning).
2) The cost of storage is falling continuously, but the size of transactions is constant. You are extrapolating a problem based on today's prices for storage.
1) The issue is about "spam" accounts that have small amounts and stay unused. If the tx out is unspent, it cannot be pruned. Pruning is not the answer to every problem with storage. First you actually need to download the whole block chain first, THEN prune. It is not very efficient. If you trust somebody else's prune, you are trusting them instead of "no one." It saves no bandwidth whatsoever and only eventually saves disk space.
For perfect security, one can download and verify each block from the network of BTC nodes. Sort of like downloading the source code, inspecting it yourself, and then building the executable yourself.
Or you could download a signed full blockchain and then verify each block and only download the last few blocks from the network. Not as secure..
Or you could download a signed pruned blockchain that has been signed by N authorities you trust (like Gavin, or the Church of Scientology, or Bill Mahr, or etc). Less secure but that would save bandwidth.
BTW, bandwidth is also growing as fast as storage. (Just not in the USA because our infrastructure is made out of feces and michelle bachman's tears). In Sweden they have tested 40Gbit residential links. Not sure when they will be generally available.
2) See the wiki about 4,000 tps and what kind of storage you'll need. I'm extrapolating; you have a narrow view.
BTC would be worth a lot more in dollars which means TX fees would be worth a lot more in dollars.
the wheels on the bus go round and round, round and round
You cannot both presume that BTC will suddenly be popular and worthless at the same time. Either BTC is in widespread use before storage becomes incredibly cheap and therefore BTC become more valuable in dollar terms since they are created at a fixed rate and cannot expand to fit demand. Or BTC is not in widespread use before storage becomes cheap and there is less of a problem with needing to store massive transaction volumes. Miners could also just ask for higher transaction fees to support storage as a stop gap solution.
Another potential outcome is that BTC could become a currency for the rich. It costs money to participate and issue transactions and mine. Only people able to support the currency can participate in it. No need for a tax designed into the protocol, you are taxed in electrons and hard drives.