A 100Mhz Pentium in 1992? More like a 25Mhz 486 if you were lucky, my friend made it run on a 286. And you'd get about 15fps, wolf3d used to give people brutal motion sickness.
Wolf3D is also an interesting example because the rendering is done in software (so your high powered HD video card doesn't help at all) and I guess a lot of development time was spent writing asm routines/hacks to take shortcuts with the necessary math.
To be fair, a lot of the reason that people are remaking these old games is because people like playing old style games and the people building these demos are probably doing it in a very small team (or even solo).
It should certainly be possible to remake Half-Life (maybe even HL2?) entirely in javascript/WebGL or make more sophisticated 3D games. But we won't see it until some bigger gamedev is willing to risk a fair chunk of change on the browser as a gaming platform.