We are looking at block chain that exceeds a Terabyte easily. Might take a year to download it if we are talking about decentralization and accommodating slower connections.
I don't believe you need to download the entire blockchain to a mobile device, and we now have some real world usage data showing that Monero (ring sig plus denominations) vs Bitcoin for typical usage is not orders of magnitude (it might be around 5x).
But putting those aside (it is certainly true that widespread adoption means much higher transaction volumes), a terabyte is not necessarily a show stopper. I get about 2 megabytes/sec on my mobile devices. That is 6 days to download, not a year. Soon this will be considered slow. I've heard talk about gigabit mobile (50x faster than mine -- download 1 TB in hours).