I guess OP means "amount of content", which is definitely true. That one little trick, combined with genuine fresh gameplay, gave it some incredible longevity.

I've been playing through the first two Thief games, released 20 years ago, and I put 60 hours into beating the first and am already 30 hours into the second.

Gotta say I'm extremely impressed given most games I've played in the last few years are on the order of a few hours' playtime.