Firstly, thanks for bringing back the burst-heal! Secondly, why even change it at all now?
_Note: These changes haven't gone on the PBE at the time of writing, so I'm just going from experience and what's written on paper here_
The root on live (ROOT DURATION: 1 / 1.25 / 1.5 / 1.75 / 2) combined with the 25%/50% slow from the Q/Mantra Q are already strong enough to keep someone in place as support/skirmishing Karma. The extra long root is pointless in teamfights (1-2s is definitely long enough to finish off a squishy target) and during laning-phase I'd much rather take the doubled slow (+AoE slow) and damage from the Mantra'd Q, so why remove the bonus damage from the Mantra W?
Karma is supposed to be able to change her power depending on the situation which is why it's so nice that all her abilities get extra damage (or damage at all, in the case of the shield) when ulting them. If the W is too weak on the damage during low-health duels it will still be better to use the extra damage from the Mantra Q when the Mantra W is supposed to shine.
I don't even know why Karma is getting changed at all, she's always been that "balanced" champion for me. Not too strong to be seen all the time, not too weak to be laughed at when picked, while having a unique kit that allows for a lot of dynamic gameplay.