Typical stuff, unfortunately. The solution is for some higher-up to take ownership of the problem. The gate agents have other flights to work so they move on leaving nobody to make frequent update announcements. This leaves the pax to beg for information from harried agents or standing in long lines at the Customer Service counters. The monitors are automated and the airline isn't interested enough in figuring out how to override it.
And BTW, having spare airplanes is largely a luxury of the past. Especially ETOPs ones, and especially West-metal ETOPs 757s.
Anecdote from a month ago. I was a jumpseater on the noon flight from PHX-ORD. Just before departure time the captain calls Crew Scheduling to inquire why the FO listed on the flight release hasn't showed-up yet. (It's contract negotiation time so we're only doing our job and not everyone else's.) Scheduling is dumbfounded. Turns out the FO had been sitting around all morning after his flight cancelled then he went home to LA. (He should've called scheduling first but I don't know if anything happened to him.) So he was assigned the ORD flight but never informed. Scheduling might've left a voice-mail but it was obviously never acknowledged. Bottom line, scheduling dropped the ball. So twenty minutes after departure time scheduling calls a reserve FO -- who has a two-hour call-out. So a new departure time is set for two-hours later.
But we're not done yet. Fifteen minutes after her check-in time there's still no sign of the FO. A call to scheduling reveals she's checked-in already. Five minutes later she shows up and we go. What happened was she initially went to the wrong airplane -- actually the next flight to ORD -- because it was the only ORD flight listed on the boards. The gate agent there knew nothing. So she had to call scheduling -- and wait on hold -- to find out the correct gate and walk there. She'd actually walked past the correct gate coming in but the screen was blank so she couldn't know.
Just one of many daily embarassments in the airline industry.