So it does indeed look like magic-immune opposition are only going to be a big problem for the first half or so of the average adventurer's career. I don't think that anyone was expecting it to be an issue for high-level parties.
No, that's not at all what I'm saying.
I'm saying that for the most part creatures with resistances and immunities used to be a real concern, but is not that anymore. Sure there can be the odd exception, but mostly, you will have gotten the tools you need by the time you face that kind of monster.
Time and time again I read about a monster's resistance to magic weapons, or somesuch, but not even once has this actually shut down the party, forcing them into changing their tactics.
Sure, it has meant a fighter have had to use another weapon than his favorite, but... meh.
The thresholds are simply set so low, that most parties will fulfill them just by regular adventuring, with no need to take detours, performs quests or make sacrifices in any way.
This should be obvious for everybody to see. There is only a single "level" of better gear to find. Either you have a non-magical weapon or you have a magical weapon. (Sure silver and adamantine are sometimes mentioned, but without any real practical impact, since any old +1 weapon does just as well)
Needing a +2 or +3 weapon? Gone!
Needing an anarchic or axiomatic weapon? Gone!
Needing a cold iron or adamantine weapon (with no substitutions)? Gone!
Magical energy based attacks are given to every first level spellcaster. And they're unlimited in all regards! Even fighters and rogues can have them (either by feat or subclass).
I see it as part of a definite trend to make D&D... easier, for want of a better word. Very much easier. In order to make D&D run like it used to, you need to tweak everything yourself. Not one of these obstacles will ever be felt by a party running official modules, assuming baseline competency in creating a flexible party that covers the necessary bases.
Look, I'm certainly not advocating for a return to the bad old days where a monster could resist 40 points of every attack unless your caddie pulls out the right club for the job. But it definitely seems like WotC thinks us gamers don't like setbacks and problems and being denied stuff, because this edition is ridiculously easy to beat.
(You know and I know I can throw a dozen Balor at a level 2 party. This isn't about that. This is about the default difficulty dial being turned down a notch or two compared to every other edition I know of, in every damn area. It's a significant and profound change that shouldn't be hidden behind conciliatory language)