I understand in theory the blockchain shouldn't be an issue. Even cryptonote (I was reading about how the blockchain when it grew to BTC size would be 10X as big as BTC). But as a consumer downloading a 20GB database is a fairly big deal with my internet connection. (I understand I can use litewallets that sync with other servers)
But isn't the blockchain size fairly relevant to keeping decentralization in the sense that bitcoin was originally invented - to be duplicated across hundreds of thousands of consumer machines? Even at 20GB - with my internet connection I really don't want to download the entire blockchain ...
Yes - as it stands it's definitely too large for consumer machines. The thing is, even something like mini-blockchain is only a temporary fix, as any cryptocurrency that scales up to meet massive global demand will find that home Internet connections (even in 5-10 years time) will simply not have the low-latency and speed required to maintain a full node. I remain unconvinced that there exists a better solution for "Visa/Mastercard scale" than off-chain transactions that are settled back on the main chain on-demand or periodically. A company like PayPal or Google could ostensibly operate something like this, although it's not inconceivable for Visa or Mastercard themselves to provide this.
I am going to offer a different perspective here namely that of a member of the baby boomer generation. Most members of my generation are not very technical but some of us are. In 1979 after completing my undergraduate degree I was working as a research assistant and wrote a program in FORTRAN that required over 2MB of RAM. I got an error saying that my program exceeded the total memory capacity of the university's mainframe computer. We are talking about a major Canadian university! Now fast forward 11 years and I bought a computer with 4MB of RAM. This was a typical consumer computer somewhat on the low end. 10 years before in 1969 the Apollo moon landing occurred, at the time one of mankind’s greatest achievements. Here is a link on the computers in the lander. Memory measured in kilobytes. https://en.wikipedia.org/wiki/Apollo_Guidance_Computer#Memory Having lived with Moore's law https://en.wikipedia.org/wiki/Moore%27s_law and its bandwith and storage counter parts for most of my life, I confident that it will easily solve the "bloat" issue for crypto-currency just as it made credit and debit cards possible. By the way would the current VISA transaction rate have been possible in 1959 when American Express and Diner's Club credit cards were launched? Not even close by several orders of magnitude.
Edit: The real risk here is artificial limits such as the 1 MB blocksize limit in XBT and the debate surrounding it. The fear of bloat can cripple a coin.