It's a Diablo game - they intentionally keep things laconic and on the need to know basis, because people mostly don't give a fuck about expositions and just want to move on to kill shit.
You literally have people who see this thing first time ever and already skip all the cutscenes let alone flavor text.
I mean, do we really need everything chewed up and explained to us in a typical "Muhahahaaa let me explain my whole grand plan in finest details to you, random dude I just met, so you can fuck it all up here and now" manner?
I think reasonable obscuring of the grand scheme is important for a better storytelling, this way you get to sit there and wonder about what is going on. Hell, I'd go as far as to say that majority of people who would play this game won't even have a clue about Sin War.
In such an environment don't expect some extensive expositions in-game about how Inarius got away/was let go - the Horadrim do not know that and that's your only credible source of reference outside Mephisto telling you that bit that all he did was "focus his anger".
Just one more 4D chess move out of zillion moves Mephisto did over the course of franchise. The whole bloody wolf helping you from first minute of the game shows just how far or roundabout his foresight and planning go.
Heck I would not be surprised if in expansion or two down the road we'd figure out his whole pokeball moment was yet another step towards the actual goal, even with all that protesting and being afraid charade going on. This is a Diablo game after all and he conveniently found himself where he likely wanted to be - in the Sanctuary.
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Yes, but he tells you literally sentence after that despite all that - it's the best weapon we got, which is true.
Soulstones fail eventually, but this eventually can be a thousand years away, as opposed you just "killing" the guy and him just reforming within your lifetime as we have seen now.
In other words - Soulstones bad, but they sure better than anything else we'd do.