OED concludes it to be "rare" now. I'm wondering if native English speakers find these quotes too dated and unacceptable for today's English.
1668 Howe - Had this been the alone folly.
1873 Goulburn - Christ is ... the alone source of sanctification
1874 Blackie - The alone keystone of all sane thinking
Cambridge Grammar of English decides 'alone' falls into the group of adjectives that are formed with the a-prefix (< prep. an/on) and is predicative only.
OED, however, says it's a contraction of 'all' and 'one', just like German 'allein'.
Is Cambridge's opinion more popular in the UK?
To me, Cambridge Grammar is obviously wrong about the etymology, unless it silently allows analogy. (it also puts 'afraid' in the same group, even though 'afraid' is a worn down form of the ppl of affray < ex-fridare).