Romero did not leave, Carmack fired him, in his own words, for "not working hard enough."

Citation: https://www.bluesnews.com/articles/carmackinterview.html

Eh, judging from the Masters of Doom book, i wouldn't take his words seriously considering the tension that existed at the time between the two and how Carmack basically burned out himself during Quake's development. Romero wasn't the only one who left id at the time.

Why the downvotes?

Quake III Arena (Graeme Devine) was a fantastic game, although Doom 3 and RAGE weren't particularly inspiring.

Doom (2016) was released after Carmack left, but it's one of the best shooters they've made.

Q3 always suffered from the fact that it was released right around Unreal Tournament which was a far superior game, especially in level design.

Its a matter of preference, I think. I was only able to play UT growing up, thanks to its software renderer, and finally played Q3 (Quake Live) in college.

Q3 is more minimalist, which makes competitive play more interesting. Q3 has also maintained a competitive scene to this day where UTs seems to have fizzled out.

Quake 3 Arena is still one of the best multiplayer shooters ever, so I can't agree with your statement.

It has great game mechanics, but in all other manners of design it feels like a hodgepodge of unrelated assets thrown together. Quake 1 had a similar problem early in development, but they managed to tie it together. Quake 3 didn't even bother with a true singleplayer campaign, which seems like a winning move in hindsight, but at the time it was a departure from the norm and I can't help but feel like it was a move they did out of necessity.

While surely being a marvel of software engineering and level design, it had nothing in common with Quake I and II. It is not my intent to be critical here, but QA3 felt like a circus, especially compared with other multiplayer shooters such as the original Unreal Tournament, which had a wonderful mysterious atmosphere to it.

I was just playing something recently and thought "This feels like an Unreal Tournament Space Castle." Can't remember what game it was.

UT99 had some fantastic level design. Facing Worlds, Phobos, HyperBlast, Deck 16, Coret, etc.

Q3 is intentionally stripped down. Primarily due to Carmack's direction. Iirc from his old plan files.