I agree with you, it's very similar to what was requested here T155142, it's unclear to me why such low boost values have been set to non content namespaces. I'd be tempted to change the default weights and set them to at least 0.9. They are currently set to:

$wgCirrusSearchNamespaceWeights = [
        NS_USER => 0.05,
        NS_PROJECT => 0.1,
        NS_MEDIAWIKI => 0.05,
        NS_TEMPLATE => 0.005,
        NS_HELP => 0.1,
];

// Default weight of non-talks namespaces
$wgCirrusSearchDefaultNamespaceWeight = 0.2;

// Default weight of a talk namespace relative to its corresponding non-talk namespace.
$wgCirrusSearchTalkNamespaceWeight = 0.25;

I'd be in favor of changing the defaults and adapt them on a case by case basis when users consider that a namespace is too noisy and needs to be aggressively discounted.

(The main problem I think is that we've lost track of the reason why such low weights were set like that in the first place)

I'll just add an agreement here. If users are searching 'Everything', then they either know what they want isn't in content, or have looked in the content namespaces. Pushing content up so highly in the everything search makes it less useful. The merged ticket above is an example where searching everything should have brought the related page with a full title match to the top, but because content is weighted so highly it doesn't show up at the top of the results.