To reiterate: the later in the boarding process you are the greater the chance of not having overhead bin space and thus having to make the choice between gate checking a bag that's bigger than a personal item and not taking that flight. The chance isn't 100% (as you've taken basic flights where there was bin space), but the chance isn't zero.

If the contents of your messenger bag are sufficiently valuable to you that the risk of damage if gate checked is unacceptable (even knowing that it's reasonably possible to pack them such that the risk is minimal, assuming you are also packing clothes), it seems likely that buying up to Comfort+ (which, assuming you board in that group, is basically zero chance of gate checking) is utility optimizing (even if negative EV, though if we're talking $4k worth of electronics, you'd only need a 10%-ish chance).

Me: I'd have no reservations checking my Thinkpad chargers. The Thinkpads themselves, I'd leave in the carryon. My fairly large LL Bean messenger bag holding a 15” and 13" laptop with a common charger, power brick, cased noise canceling headphones, and a day's change of clothes also fits under seats just fine.