> Every Flash/Shockwave browser game is becoming increasingly hard to play.
If you have the actual .swf file, you can run the game in Flash Projector, easy!
> Games for ancient Windows versions can be devilishly hard to get running
What doesn't work in Virtualbox? Luckily, games from the 90's generally don't need GPU acceleration. I'm also continuously amazed by how much just works in modern Windows.
> games for PowerPC Macs are all but impossible.
There you have a point. Although even then, you can use VMWare + some unlocker tools to install Snow Leopard, and from there use Rosetta. Qemu is also supposed to be pretty good these days, although I've never tried it. Alternately, it's not that difficult to track down old Mac hardware.
> Emulation is flaky and incomplete for console games of all kinds.
Huh?
The Atari, NES, SNES, Genesis, Playstation, and all Gameboy models have damn-near perfect emulators. Identical to console down to the pixel, for every game.
Dolphin isn't quite take-a-microscope-to-the-screen accurate, but it will run the vast majority of the Gamecube and Wii's library such that you won't notice a difference.
The N64 and PS2 lack great emulators, but what's available is still very good. Some niche titles will exhibit glitches or refuse to run, but most stuff works well enough.
The Wii U and PS360 don't have such good emulators yet, but that's because those consoles are relatively recent. RPCS3 and Cemu are making great progress, and can already run a handful of large titles without problems, such as Persona 5 and BotW.
The original Xbox lacks a usable emulator, which sucks. Luckily, this isn't the norm.
Emulator developers have done amazing work, and the result is that most of gaming history is fully open to your exploration. Games will never be quite as plug and play as music files, but they aren't that labor-intensive to get working either.