I mean, I'm certainly weighing this season disproportionately for Murphy, but even if we assume his offensive game didn't blossom (in terms of generating team chances while on the ice), and that this is simply a fluke year, he's essentially Manson (with respect to defensive outcome) anyway.
I actually really like Manson, despite some less than flattering comments from others in this thread. He's exceptional defensively (even this year), and he faces difficult competition while receiving minimal offensive zone starts. My contention is that Murphy, similarly, faces exceptionally difficult usage, and has performed very well both offensively and defensively (while Manson is essentially an offensive black hole).
I guess it depends on what one believes the Leafs need. I want a defenseman who drives play at even-strength, and limits chances against. I don't believe the Leafs are in need of a high-scoring defenseman (or one that is even capable of it, frankly), but they do need a defenseman who can transition the puck up the ice; Murphy does that better than Manson.
I have absolutely no interest in Dumba. Maybe he finds his game again, but even if he does, he's not the type of defenseman the Leafs should be interested in. If he doesn't? He'll make Ceci look good.