But Janes is a long established character with ties to the Thor mythos. And the hammer inscription quite clear states anybody "worthy" can wield it. Doesnt say they need powers.
I see no problem with Sam being Cap. Like Jane, he's a long time partner and friend of Steve. Steve was incapacitated, so Sam took up the shield. How does that not "flow?".
And Miles didnt replace Peter. Both have their own book. As do Sam and Steve.