This is still a buff even for Festerblight. Scourge Strike far out damages Festering Strike as it is, and this is just going to enforce that more.
Festerblight only works because the damage gained from keeping higher-DPS diseases is greater than the damage lost by dropping Scourge Strike in lieu of more Festering Strikes.
With Festering Strike cut by 10% and Scourge Strike buffed by 20%, the opportunity cost
of using dropping 2x Scourge Strike in favor 1x Festering Strike is too high to make it worth doing, even if the absolute damage for the festerblight rotation goes up.
Festering Strike on live is 200% weapon damage plus 197.
Scourge Strike on live is 135% weapon damage plus 173, and 50% of that amount as Shadow.
In order for Festerblight to be better than a normal rotation, the increased disease damage needs to match the difference of 70% of weapon damage plus 149, with 50% extra as Shadow.
Come next patch, FeS will be 175% weapon damage plus 172.
Scourge Strike will be 165% weapon damage plus 211, and 50% of that amount as Shadow.
In this scenario, in order for Festerblight to be better than a normal rotation, the increased disease damage needs to match the difference of 155% weapon damage plus 250, with 50% extra as shadow.
I don't see any patch notes that show disease damage increasing by a huge amount, ergo, Festerblight - which is already barely a win - won't be. We'll still want to try to refresh our diseases at high damage values, obviously, but it won't be worth sacrificing Scourge Strikes to do it.