I really don't get what are you trying to argue over this thread. Like... at all. I mean, there is no such class in D&D that is completely tied to a single elemental attack to contribute in the combat pillar. No elemental-benders, or pyromancers or such. Even if all you want to play is a elemental-themed dragon sorcerer (which they really are not), you should already be choosing at least half of your spells as non-blasting just not to feel too bored doing the same thing over and over again (and also so to not gimp yourself too much, as damage-dealing is a very subpar activity for a full caster).
Non-caster martial weapon-users, on the other hand (such as champions or rogues), are pretty much "married" to physical damage. And while some odd monsters have resistance/immunity to one or two types of weapon damage, most that have resistance/immunity actually resist all three types. And then, for these classes, there is no in-class workaround. They either found magic weapons or have to count on their spellcasting friends' goodwill to enchant their weapons. And then, there are those other monsters against which having different damage types is a real boon (like fire to trolls and treants, or radiant to vampires)
To address your point in a more straightforward way, having resistances, immunities or vulnerabilities in the game can be interesting or lame. The problem is not really in the mechanics, but in the way they are deployed by the DM. To face an unknown enemy that you had absolutely no opportunity in game to gather intel, and have the final battle be a big gotcha as you throw your most powerful spell just to find out the monster is completely immune seems like a work of a really jerk or incompetent DM. But to face an enemy that you could have learned something while exploring that lost library in the dungeon, or by interacting with the scared townsfolk who survived its previous attack, and getting properly ready for the final confrontation is awesome. And being able to ignore its resistances/immunities just by playing the system, as opposed to playing the game, would somewhat undermine this awesomeness. And finally, if the heroes had solid opportunities to figure out an enemy's strengths and weaknesses beforehand while exploring/social-interacting, but blew these opportunities, and now the fight is looking bad for them, good thing. Because the other pillars can (and really should) be that relevant.