Well it's clearly not working. None of the current talents really let you make playstyle choices at all.
When S2M was a thing, much as I hated it, I was still stuck playing it. Even now, even though I don't really like having to go TOF/LI/AS/MB/LotV every single fight - that's the clearly correct choice.
If they want us to choose our own playstyles, they need to make two paradigm shifts in their development approach:
1) Put fundamental choices of the spec into the talent trees. Voidform should be a talent alongside maybe S2M and maybe old Devouring Plague. Shadow Orbs vs. Insanity vs. No Resource At All should be a talent tier. Choosing between 3 basically passive abilities is not a meaningful decision to playstyle.
2) They can't be Blizzard Entertainment. A company that does one patch to balance talents every second year (every new expansion) is not at all capable of making competitive talent tiers. If they want competitive talent tiers - they need to take the Mythic Entertainment development approach, and adjust things by like +/- 2% every week - with so many patch notes that only the hardcore nerds can even keep up - but everyone else gets to safely assume all talents are competitive.
Edit: Also, there is another major development approach that prevents them from achieving that goal. Secondary stats. The biggest decision we make about what playstyle we are as Shadow, is actually buried in our gear selection currently. Haste makes us good at single target. Crit makes us good at multidot. Versatility makes us more survivable in difficult encounters. Mastery makes us bad.
If we were really selecting playstyle choices, the importance of secondary stats should be like a talent tier all it's own, example:
Single Target Talent Choice: Your GCD is reduced by 0.5 seconds, and your Mind Blast cooldown is reduced by % haste, and instant cast. Highly rewards stacking haste gear for single target fights, and adjusts how you approach single target encounters - making your DoTs less important, making your Mind Blast cooldown extra key.
Multi-DoT Talent Choice: Your Auspicious Spirits proc from all periodic critical hits and generate 2 insanity on proc. Creates a positive feedback loop where more Crit Chance = more DoT crits = more Insanity = longer Voidforms = more DoT damage (Mass Hysteria) and Haste (more DoT ticks) = more DoT crits
Mastery Talent Choice: Your Mastery is only affected by every second Mastery nerf, so that it's not a completely dead stat.