Backpack vs Messenger Bag?

I've been getting away for a while with a 15" Bellroy Totepack Duo in some sort of dark grey color scheme – they don't make this exact model anymore, but: (a) they still make similar ones; (b) I'm sure eBay / Amazon still has them.

Initial rationale: I can't stand leather bags / briefcases in all shapes or forms. The bag has a standard-issue-ThinkPad-bag vibe to it, so professional enough for me. Plus I really like the portrait orientation of it, but that's my quirkiness showing through, I reckon.

Pros:

• Can fit all you need + extra. I typically pack my gym stuff (save for sneakers and flip flops that I store at the locker that I rent at my gym), an old thick-ish Lenovo f--ker of a laptop with a mouse and a charger thereto, a notebook, a spare shopping bag, sunglasses, a wallet, a research report to read on a weekend (which never happens, but hey no harm in trying), some other small random stuff – and there normally is still is a fair bit of free space left. I've even taken a weekend trip with it once (albeit without an aforementioned Lenovo).
• Semi-detachable shoulder straps. Almost no visible signs of their existence when hidden. Helps A LOT to have an option to convert it into a backpack when: (a) it's is heavy for some reason (e. g. carrying a bunch of materials); and / or (b) you are more casually dressed so you might want to "dress down" the bag as well; and / or (c) you are on a bike or powerwalking.
• Durable. I've been using mine (business) day in, day out for the past 1.5 years, and it displays no sign of wear.
• Lots of pockets of all shapes and forms. No external pocket(s) for water, which I like – I'm not on a hike ffs. Internally, has 2 cushioned sections for laptops + another one for an iPad.
• [Might not be relevant for you, as this feature is not present on current models.] There is a feature that helps to slim it down when you don't need all that extra space.

I don't insist on 15", it's just my laptop size. If anything, ordinary (non-Duo) 15" one might look a bit chunky, so I would've gone for a 13" myself if I could.

I know it's not the most direct answer to your question, but hope this helps anyway.