I was quite active here at that time and I don't think your assessment is correct. I agree when you say "every aspect was debated" - that's always the case. However, take a look at what aspects the discussions focused on and how the polls of that time ran. I think
sgrig made a good comparison.
The vast majority of complaints about Civ4 where:
1. the game doesn't run (technical problems)
2. stupid unnecessary 3d engine doesn't let me run the game on my machine
3. inefficient engine doesn't let me play huge maps
Even the "Civ3 > Civ4" crowd mainly cited these reasons, not any perceived "superiority" of Civ3 mechanisms. I don't recall anyone calling for a reintroduction of Civ3's corruption system. On the contrary, even the Civ3 crowd said that corruption as implemented in Civ3 wasn't enjoyable, usually their position was that corruption could be modded out of Civ3. T.A. Jones advocated a patch which did that in every discussion.
To a lesser degree, there were also gameplay discussions. These focused on 2 main aspects:
4. The game punishes warfare too much
5. Implementation of artillery as kamikaze units makes no sense
Of all these criticisms, only (5) survived for a long time. The technical issues were remedied with patches and/or resolved themselves as users upgraded from old hardware. Issue (4) was resolved by patches which lowered city maintenance, and the Warlords expansion added more options for warmongers.
Yes, but most of these arguments and threads focused on the
presentation of the game, not the gameplay.
Of course there
were claims that Civ4's gameplay had also been "dumbed down" - a good example is that from time to time, someone came and complained that the reduction of the distinct offense/defense strengths for each unit to a single strength value in Civ4 was a sign of "dumbing down". But, well, that assessment obviously didn't have much merit since the whole promotion system was added to allow units to be fine-tuned, so these discussions died out pretty quickly.
Another example that shows the difference are the AI discussions. At Civ4's release, most discussions regarding the AI revolved around someone claiming that the AI cheated (who was then usually proven wrong). For Civ5, the majority of AI complaints is that the AI can't handle the game's combat at all.
Really - I've seen game launches since the industry exists, and as far as forum discussions go, Civ4 was - as far as its game design was concerned - one of the most widely accepted evolutions of a franchise that I've ever seen. Civ5 is seeing a pretty divided fanbase, comparable to the release of Oblivion (another game that diverged from core gameplay elements of its predecessor, was harshly criticized by many, but also had a substantial number of players who liked it). The fan reaction to Civ5 is
not as bad as the fan reaction to MoO3 or HoMM4 was though.
I really don't see where the claims that Civ4 had a similar forum reaction than Civ5 are coming from. It looks like an attempt to rewrite history, but it might just be selective memory as well. In any case, the whole premise doesn't make sense - Civ4 was a clear attempt of an evolution from Civ3, while Civ5 is a clear attempt to produce a very different game that's still Civ. It was absolutely predictable that Civ5 would have a worse reception among the fans than Civ4 had (this was then exacerbated by the fact that the game doesn't work very well right now, but patches may be able to remedy this).