With the death of Steiner the film would have been an even bigger downer after almost all the Platoonmembers got killed.
As for the ending:
Like during STRAW DOGS, Peckinpah improvised it.
There's this famous story that Hartwig and the other money men pulled the plug on CROSS OF IRON while
Peckinpah wasn't really ready to wrap. He sat on a wooden tower and cried. Coburn got very upset and angry that these suits brought out the tears of his beloved Sam, so he pushed the guys off the set and they spent the reminder of the day shooting like crazy to get as much as they could.
That's true, sure.
But one has to understand the whole here: first, Peckinpah wasn't very disciplined in terms of time (=money). When he shoot, he wouldn't stop until you just sent the boys home. When he edited, he edited until you kicked him out of the editing room or a new film started shooting. That's a major pain for producers, on the other hand it's a reason why his films are better than most films of his fellow directors.
I think CROSS had ovr 90 days of principal photography! Poor Hartwig had to sell parts of the film to EMI in London. Then again, previousely he thought he could handle Peckinpah (wrong guess).
The film starts different than the script (which starts with Stransky) and the ending (as for example the 'walk thing' in WILD BUNCH) is a couple of lines. Peckinpah's vision for the complex end-montage asked for three shooting days!
If you read the attached script excerpt, you'll understand why a producer would say 'A day for this is more than enough!!'. They were over over budget anyway.
Anyway, there are film makers who make great films from good or great screenplays. Peckinpah was one of the few, who could produce magic on screen based on three lines

. But for this magic he needed many set-ups (contrary to Leone), which takes a lot of time, especially when dealing with action.
Steiner steps close to Stransky. With his free hand, he
pulls a grenade from his belt.
STEINER
Stand close to me, Captain.
STRANSKY
(backing away)
No -- No -- You can't --
STEINER
(pulls Stransky
to him)
This will do for the both of us
-- the final fraternization --
STRANSKY
(a whimper so low
it can scarcely
be heard)
Steiner -- please -- Steiner --
Steiner pulls the pin from the grenade, holds it between
himself and Stransky.
STEINER
What's the count before it goes
off?
STRANSKY
(his lips move, but
no sound comes out)
-- Steiner --
STEINER
Count with me, Captain -- One --
Two --
The CAMERA STARTS to MOVE UP and AWAY from Steiner and
Stransky.
STEINER'S VOICE
-- Three -- Four -- Five -- Six --
The CAMERA is at TREE TOP LEVEL now -- Suddenly there is
a SHATTERING REPORT. The branches of the tree tremble.
The CAMERA STAYS on the tree for a moment. The leaves
stop trembling. There is a vast silence now. The CAMERA
starts to PULL AWAY AGAIN. As it DOES:
FADE OUT:
THE END
The actual ending of the film catches of course the spirit of the written word, but it's much richer,
cinematic, inspired and memorable.
Peckinpah even included an idea he couldn't use in his previous film: in THE KILLER ELITE Sam shot
a scene were the dead Bo Hopkins sails into the sunset (final take in the film, but without him) with
Caan and Burt Young. United Artists cut it out, they though it was weird.
Working in Europe he had more freedom, that's why the blond russian boy was brought back to
life in CROSS. Great stuff.
