Video game endings weren't exactly in any sort of renaissance when this one came out.

You were lucky to get any kind of ending. Just about every FPS around this time and soon after was convinced that they'd be the next mega-hit, so they were often "to be continued" or in some cases cliffhanger endings that were never resolved since (as is natural) most of these companies went under.

That's the end for beating the first episode.

And yeah, it's certainly better than the first episode of "Doom," where you take a teleporter into a room filled with monsters, get mercilessly slaughtered, then have a paragraph of text dumped on you that amounts to "there's more."