I’m pretty sure it’s a statement, that’s why we (for example) set both the begin and end year to 2017 for © or ℗2017. Not to mention representing the span is very difficult as it varies by country and changes fairly regularly.
Not really sure how that follows from it being a statement of copyright. Releases can state they claim copyright on the compilation (thus on the release) or on the individual recordings (thus on the recording) or, even, on both.
Saying “whatever! we don’t care! Multiple dates is ugly! Put it on the release” also doesn’t work. For a fair number of releases, different tracks have different ℗ dates and companies. Having a pile of ℗ dates and companies on a release without any way to tell what tracks they belong to is far less useful. So essentially then we’d be left with using the release annotation for any hope of accurate information—and that’s also far less useful, as its not standardized or easily machine parsable.
Take a look at https://musicbrainz.org/release/db77f67d-9e9b-4623-8ea1-baab173cbe49/cover-art — the booklet gives ℗ dates and companies for each track on 19 CDs and then a separate ℗ date and company for the compilation. By putting track ℗ on the recordings and the compilation ℗ on the release, that is essentially all represented in the MB schema (the only thing not represented is which of the dates is the digital remaster). Your proposal would have (quick guess) 40+ ℗ relationships on the release and convey significantly less information. It would also mean that the work that went into entering that information can’t be shared with other releases of the same recordings, even things like https://musicbrainz.org/release/2546da4f-9402-4ada-9426-3a79b8ed2e5e which share the same discs (down to disc ID and content, at least).
Copyright law considers anything that has some minimal level of creativity in it a new work, eligible for a new phonographic copyright. Each of those typically only has one date (though if its worked on by one person/company over the course of two years it may wind up with two years). But at MB, we consider all those different (from copyright’s perspective) works as one recording. So a recording will naturally end up with multiple ℗ dates, potentially from many labels. So even with first-date-only, you’d still get a bunch of dates/owners on the recording. (There are more reasons to get multiple, of course, that’s just one).