I don't know much about the old Syndicate, but, to be fair, the complaints I remember most old school fans having about Fallout 3 weren't exactly First Person based... Most of that crowd accepted New Vegas pretty well (while the mainstream had the opposite reaction to both).
The people who whined that Fallout 3 didn't have the same writing quality, world cohesiveness and internal consistency of its predecessors weren't wrong. They just forgot that most of the world doesn't care about that stuff.
I'm willing to bet Syndicate failed because it was a boring shooter, not because it deviated from the original. I'd guess the Xcom first person shooter was canned for similar reasons: Aesthetics aside, it seemed like a boring shooter.