As for STID, if Kirk had failed then everyone could have lived - escaping in life-pods. If Sulu hadn't decided for the rest of the crew not to go that way.

'Only' the Enterprise would have been destroyed. So Kirk didn't 'need' to sacrifice himself in STID.
Spock in TWOK did as escaping in lifepods or shuttles wasn't an option.
Kirk would have no way of knowing that lifepods and shuttles would not be shot down by the Vengeance. He made a decision, based upon the information that he had at the moment. It doesn't change the nature of his sacrifice or the impact that it had upon his character arc.
Didn't McCoy tell Kirk that the Vengeance had been disabled?
Anyway Kirk should have contact the bridge before shoving himself in the WarpCore. There might have been other reasons why that might have been a waste of time.
Its actually not a big deal IMO. Just another place where the writing could have been a bit finer.
I means we get some great scenes where Sulu, Spock and Kirk are all very brave and I agree just because it was somewhat unnecessary it doesn't change the nature of Kirk's sacrifice.
If they were being realistic everyone should have abandoned the Enterprise after making certain she would burn up in the atmosphere. People are more important than machines IMO.
If the writing was a little more cleverer then there might have been some valid reason for Kirk's sacrifice like half the crew were trapped in Engineering or something or the lifepods were inaccessible.