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Damn, even by saying pages that don't generate traffic now won't much more in the future, and by giving an educated estimation of 0.05% potential future gains by keeping them versus the boatload of progress it could mean for the website to noindex them, it couldn't convince the webmaster to cut them out of the index...

Anyway thanks for your help everyone !

The less you put in robots.txt the better...

If there exists anywhere (on your site or on another) a link to some page that you put in robots.txt, it won't stop google from indexing it, and it will look bad (no title, no description since the bot can't access them). Worse, if you wan to noindex that page in the future it won't work !

So block content if you know there's no link to it and there won't ever be, otherwise the right move is the noindex meta tag.