hell really isn’t balanced around max synergy skills. People are just used to the kill speed of a max synergy build against non immune monsters.
You can easily spec into 2 dmg sources to deal with immunes, everything will just take longer. But it’s viable in p1.
Also you got 3 respecs before finishing the game. That’s 3 more than we had in the old times of D2 and we still were able to beat the game.
And guess what, if you are not able to do it alone (because you want your MAX DMG solo element build), you can play multiplayer and play together to kill those awful evil mobs.