Una's longterm future.

[Odo]

Just because we don’t see her mentioned after The Cage/Menagerie, doesn’t mean she has to die. Maybe she goes on to be one of the greatest captains in Starfleet history or something - she’s on the recruitment poster!

As for La’an, was there ever a TOS security officer? Maybe she stayed on after Kirk became captain as security chief but was just out of frame in all the original episodes!

 

[fireproof78]

As for La’an, was there ever a TOS security officer

Lt. Commander Giotto.

 

[Noname Given]

Lt. Commander Giotto.

And Ensign Garrovick.

 

[Oddish]

Though no one is safe except the people who were alive in canon later on, I wouldn't say Una's future has to be doom and gloom. Maybe she gets her own command at some point. It's concordant with her being on a recruiting poster.