The longer vocal announcement is the right thing, at least for me (and I have a typical mind, right?). The attention passengers” part gives me a second to stop thinking about whatever and start attending to the message. But you’re right that there is no need for that with a written message, since that can be attended to even after a delay.
Beautiful!
One possible benefit to switching between an "attention passengers" screen and a "alewife train is now arriving" screen on the sign is that motion on the sign offers reassurance that the sign isn't frozen with old data up. This is probably a rare enough case to not be worth the massive amounts of cumulative wasted time over many passengers, and could be solved entirely by adding the time of the message. (Alewife train is now / arriving 12:54).
A few months ago I complained that the text version of the MBTA train arrival announcements include a not-needed-for-text "attention passengers" which delayed the key information to the second screen. I'm happy to report that they've fixed it!
While the vocal announcement is still "Attention passengers, the next Red Line train to Alewife is now arriving", the text version is the simple "Alewife train is now arriving". They've only fixed it for the second of the two announcements, when the train is arriving instead of when it's approaching, but it's still great!
(They actually changed this a few months ago, mid-April, but at the time I was waiting to see if they would fix it for the other one, and then forgot to write about it.)