The reality of the situation is that this is the democratic process. You don't get to choose who stays in or out just to bolster your favoured candidate, and shield her from (often justified) criticism, to save face or remain stronger against the opposition when we're still a very long way away from the GE. There's no reason for Bernie to bow out until it's mathematically impossible for him. Key word impossible, not improbable. In a way, Bernie staying in is much like a starter, practise for Hillary for when it is the GE and the Republican's unite on the attack.
That is unless some completely unforeseen scandal or something comes along and takes the wind out of Hillary's sails, and propels Sanders as the defacto choice. Highly unlikely of course, but just one other random possibility.