But even if you arrived after T-15, if boarding is still in process, they can't just assume you haven't arrived.

Maybe you want to sit down in open space because you've been on a long flight, and you're about to get on another one.

I've had two experiences on another airline where I showed up to the gate towards (but not at) the end of boarding (and well before the official cutoff), they scanned my BP, and I'd been offloaded for bizarre reasons (in the second case, the reason I even went to the gate that early is because I'd seen what they'd done while sitting in the lounge). In both cases, I didn't even have to ask them to go kick back whomever they'd upgraded. They just did it.

In no world does it make sense to offload a J pax before the official "boarding ends" time (or even before boarding is otherwise complete - you might just be standing at the back of the line) and fill their seat with an upgrade.