"Then what would be the legal status of of the legacy public domain base?"
It would be hard to generalize about this because of so many factors. What
country is it public domain in and why (copyright expiration? Because the
author abandoned it (and maybe other countries won't recognize that)? Unless
it's very old it's probably not free of copyright restrictions on a worldwide
basis.
For example to quote from
https://en.wikipedia.org/wiki/Wikipedia:Public_domain:
"However, some countries make exceptions to this rule. A notorious case is
Germany, which has had a bilateral treaty with the U.S. governing copyright
since January 15, 1892. That treaty, which is still in effect, defined that a
U.S. work was copyrighted in Germany according to German law irrespective of
the work's copyright status in the U.S., and it did not contain a "rule of
the shorter term". In one case, a German court therefore decided that a U.S.
work that had fallen into the public domain in the U.S. was still copyrighted
in Germany in 2003 in spite of ยง7(1) of the EU directive."
"There is an immense database of works without even a trace to their
origins."
Yes, automatic copyright means that orphan works are a problem:
https://en.wikipedia.org/wiki/Orphan_work
And the continued retroactive copyright terms that the U.S. keeps doing don't
help this.
