Well, there are different ways to deal with Immunities.
One is the option you point out, to break the Immunities. This is what Conviction does, and what Lower Resist does.
Another option is to have multiple damage types. For example, that’s what the Fire Druid has with both Physical and Fire damage.
Another option is to be more flexible in your build and have multiple elements. For example, a Hyrda/Orb sorceress.
That said I think there are a few places things could be improved.
First, I think they ought to allow Cold Mastery to pierce Immunes, that was obviously intended since there are several Cold Immunities that still wouldn’t break even with level 40 Cold Mastery. Just remove the Glacial Spike synergy from Blizzard. There, now Blizzard Sorceresses do 25% less damage, but they get to pierce Cold Immunities and so are a lot less limited in where they can farm. Maybe a very slight increase of the other Blizzard synergies to compensate (test in a PTR).
Second, they should change some synergies around so that multi-element builds are more viable. Specifically, instead of Frost Nova having two synergies it should have only one synergy: Shiver Armor. That way, you could build a Nova/Frost Nova hybrid. Frost Nova would have lower damage than Nova, but it would be a viable two-element build.
Another tactic along those lines, reduce Hydra’s synergies to just one synergy instead of two. That would allow Hydra to be combined with another element.
Another way to approach Fire Immunities for the Fire Sorceress would be to follow the Druid example, and make Meteor deal Physical as well as Fire Damage.
Or you could give Inferno/Artic Blast an ability to pierce Immunities.
The last idea has the advantage of items with Inferno/Arctic Blast available as O-Skills.
I don’t mind there being Immunities, but I do wish there were more options on how to handle them besides “get an Infinity.”