You know what's funny? After running 5e for a long time now and just "magic weapons bypass it all," I actually started re-implementing some of those old 2e/3e immunities and resistances. I wanted the players to have to think strategically about threats, to sweat if they didn't have silver against lycanthropes.
The problem I found is that if something's resistant, the players don't look for an alternative. They just hit it more, and if they lose because they were only doing half damage... well they don't think there was anything else they can do.
Once something is immune, then they start thinking about silver or non-facetank ways to deal with the threat.
As to the other point of juggling weapons mid-combat, I can see the fictional fantasy of a hero smashing a foe with a big mace to knock them over, tossing the weapon aside and leaping atop the downed enemy to stab them with twin blades. It makes me think of a Marvel movie, but I'm sure there are suitably action-ey fantasy movies that've done something like that. The idea of it definitely bothered me originally, but I can see how it's reasonable to some when I imagine it like an action movie... and action movies are definitely the vibe they're going for, I think.
And yeah it looks like WotC themselves have asserted, according to the stuff posted in this thread, that it is by design.