before you reach that limit you'll upgrade to 50MB and quantitative ease another 10 million btc into existance to cover miners - that's how it'll end up. (not really because people won't even use the 20MB version)
What plausible reasons can you give for either of those assertions?
Why *wouldn't* people use the 20MB version?
Why *would* the network majority agree to create another 10 million BTC?