The privacy implications here are in regards to stuff like Google stripping search terms off of urls from competing services. Caring about that doesn't mean you will necessarily care about hiding search history from your partner. Even with syncing passwords -- both of my parents know how to log into each other's Gmail accounts. Anecdotally, that is relatively common for older generations; sometimes they'll even ask each other to check their email if a computer isn't nearby.
If you have sync turned on in that scenario, you are going to end up with bookmarks and passwords that randomly disappear depending on who was logged into what when they were added.
And sure, you can get around that by just not using sync. The privacy implications are still worse in that situation, because you no longer have a choice not to log into Chrome. But even if you don't care about that, it still seems like a strict downgrade in functionality. You lose the ability to sync bookmarks between computers and you lose the ability to easily share logins.
The response to, "hey, you made this feature less useful!" probably shouldn't be, "well, but you won't notice if you just stop using it." It's hard to make that sound like an upgrade.
Why would they use the same browser logon. Sure they might use the same be, sure, they might even not use a different user account on the computer, but your are suggesting that they would use the same Google account to log into the browser yet different Google Accounts to log into Google.
That seems improbable, and likely to have unintended privacy implication that account consistency would mitigate rather than exacerbate.