Hardcore is an important metric because it represents players starting out with 0 rxp, 0 past lifes and for the most part no tomes.
How I build is greatly impacted by how much extra power my character has.
Last season I focused mostly on ftp accounts and played casting, melee and ranged builds to 20+. Casters are definitely weaker on the journey to 20 although they are still useful in groups. My bow ranger probably had better single-target dps than my paladin, but paladin was still overall the best character with higher hp, prr, mrr. Casters improve quite a bit once you get to 12 and the ability to pick people off with instakill + cc is very powerful for groups at higher heroic levels. I love artificer on a first-lifer, but that wasn't an option on my ftp accounts on hardcore as my favor rewards don't transfer over.
Once you get to 20 casters improve significantly on hardcore with the ED system and it's not super hard to buff up defenses with a shield and splash in unyielding sentinel. There is enough low-hanging fruit that once you get to 23 it feels really good playing a caster.
Since last hardcore season melee hp has increased and the most common casters (fvs) had hp nerfed. Some spellcasting was nerfed during the season.
Spellsingers are probably the strongest caster right now for heroic leveling, but not exactly a defensive powerhouse. I expect to see quite a few spellsingers on hardcore.
What people build on hardcore for season VII matters. It's certainly not the only metric, but it is still useful info that shouldn't be ignored.
It's a mixed bag right now. Sorc and Alchemists can bypass immunities which is really too powerful and takes alot of the art out of playing a caster. The ability to swap on a shield and get big defenses with a small US splash is probably too good - your dps sucks with a shield as a melee.
Spellsinger slas are really good - I mean it's definitely a heroic easy-button right now. And it scales really well to 32 so it's not just a heroic build.
Wizard and Warlock lag behind but do well 20+ because the epic destiny really works well for casters in general. My warlock dps is almost entirely epic spells and abilities. Still if you compare that to melees lagging behind - handwrap monks - and it's no comparison. You can't fix monk with epic destinies the way you can fix wizard and warlock. I am very happy with my assassin and don't in any way feel weak, but the ED system left many assassins sad, especially solo players that lost alot of mob control abilities.
To the extent there are adjustments it should be very targeted and not overall caster nerfs. Major nerfs aren't what's needed. The main problem is that people don't value the high single-target dps as much as they value aoe damage. Alot of that has to do with recent quest design which is pack of mob after pack of mob.