That's sort of how things ended up, but I don't think it was the intention. Doom and Doom2 were (and are) magnificent single player and co-op games, if you like that sort of thing, which I do (Doom was my lode star when I was working as a gameplay programmer / designer on Soldier of Fortune).

The better single player levels in Quake are actually really intricate and well-designed single player levels too, in exactly the way that Doom levels were great.

Some of the enemy design in Quake is actually pretty good, too, with sharply distinguished silhouettes between enemies, and good discrete gameplay property differentiation between them - well, at least for the zombies, the fiends, and the shamblers. Some of the others are fine, too (scraggs, grunts).

I think the bigger issue is they bit off way more than they could chew.

Specifically, Doom's single player mechanics thrive on having hordes of enemies interacting with highly interconnected levels in interesting ways, making space management a big part of the single player game play. Trying to figure out where you're safe to pick a fight, or how to steer the hordes around to create a space where it's safe to fight, is a big part of the draw. But, because rendering 3d monsters was so expensive compared to 2d sprites, Quake couldn't do hordes when it shipped, and fighting just a few enemies at a time (with their health highly jacked up) was a totally different experience. It could have been made to work, but they would have had to stray a lot further from the Doom gameplay recipe than they did.

And they also would have needed, I think, a lot more monster variety. A single player game with 30 different enemy types that were interesting and differentiated would have been much stronger.

That's my two cents, anyway.