i'm going to say the reason that minecraft steve looks like That is because hes gonna be like. an old washed up guy. he got sent to minecraft many years ago and kind of gave up on escaping so he got old and depressed, but now with the help of these young folks, he has found the motivation to mine and craft once more
he won't leave with them he'll stay behind to like. help out minecraft world with the rest of its problems or whatever
the post credits scene is that hes gonna be doing some shit and then fuckin. ScarJo shows up and steve will be like "i thought i was the only one here... who are you?" and scarjo will be like "me? heh... im Alex" big dramatic orchestral swell like it means anything