Best way to rebook when flight delayed (but not yet canceled)?

I was on IAD-BRU when a rolling mech delay meant I would definitely miss my connecting flight to VIE in BRU and I feared flight night be canceled (ended up going with 4 hour delay). They were letting people off the plane, so I called reservations and the agent was helpful and got me back up flights via the late IAD FRA flight that worked for me. He then asked me if I was off the BRU plane yet and I told him no, since I wanted to be sure I had confirmed flights before leaving the BRU flight. He told me I had to be off the plane, so I got off and was able to book the same flights in the lounge (and ended up arriving in VIE earlier than I would have via BRU). But I wonder whether anyone has advice on what to do in that situation. Given the BRU delay was rolling, I was concerned if I got off the flight without a confirmed backup the BRU plane could leave without me while I am trying to get rebooked and UA could say they don’t need to get me to VIE since I voluntarily got off the plane.