Visual bandwidth: I cannot relate, I read just as fast on any screen as on paper, with no noticeable eye strain.

Pictures?

Compatibility: how many old books you own are not available in digital form?

I don't know. I shall check.

ETA: I have checked. Of the last 30 books I bought (a number decided by "ok, that's enough"), 13 are available as e-books (determined by looking them up on Amazon). Every book in the sample published since 2010 was available on Kindle; only two books published before then were (2002 and 2006).

VincentYu mentioned 1DollarScan, a service for (destructively) scanning books to PDF, but transatlantic shipping costs for a thousand books, plus scanning at $3 per book make it rather expensive for me to make a serious dent in my book stacks.

Presumably, as formats change, the books get converted.

That's a large presumption. Electronic documents easily die of obsolescing formats. "If it doesn't survive, it wasn't important" is not a good rule -- ask any historian.