Speaking as someone who has had a (custom) courier bag containing my laptop stolen I'd say that if someone's going to nick your laptop they probably aren't going to bother with the niceties of taking it out of your bag in order to do so. If anything, a locked canvas bag is surely going to activate some tealeaf "lightbulb over the head" moment: it's locked, ergo it must contain something valuable.
Speaking as someone who has had a (custom) courier bag containing my laptop stolen I'd say that if someone's going to nick your laptop they probably aren't going to bother with the niceties of taking it out of your bag in order to do so. If anything, a locked canvas bag is surely going to activate some tealeaf "lightbulb over the head" moment: it's locked, ergo it must contain something valuable.