Sonic has some of the best support in Fatesinger. Arguably the best tier 4 core it turns your shouts into massive heals allowing you to function as both DPS and Healer. Also adds excellent damage to shouts and sonic blasts and Master of Music is one of the few master feats that really performs well.
You do touch on a key point though when Shiradi was nerfed, Draconic became the primary choice for most casters. Shiradi should never have been nerfed, at least not in an across-the-board 20%+ reduction. All 20% reductions are bad ideas and only lead to people fleeing from the choice and going to something else. Draconic was essentially forced upon us. I also agree that the EDs offer too little in attractive epic spells for casters. Primal has some things going for it but these are mostly at the low tiers and the Destiny falls behind Draconic as you go up the tree.
Cold and Fire are popular choices because of game design around items. These two elements are by far the most supported with items and filigree. If we had items in the game that had synergy with say, Shadowdancer casting, we'd see more casters in Shadowdancer. This is more a question of loot design and not class design.
Back to the upcoming update, I feel that if casters must be nerfed, the proposed changes are acceptable. They are only going to affect a small subset of gameplay (R7 and above) and they are moderate and reasonable compared to previous changes made to things like Shiradi or IPS.