By that token, all event buildings are worthless as without upgrades they will eventually fall behind for most players.
If you want to look at O&G let's look at O&G. Maxed out mermaid building will produce about 7.4K of T1 a day + 340 pop + culture. L19 Elven planks manufactory boosted to the max can do 5.7K T1 a day with 6x3hr collections (pretty aggressive), with 5x2 footprint and it needs more than 1K pop + culture.
So T1 manufactory takes less space by 6 squares. To equalize with the mermaid building, you need in 6 squares: provide extra 1.7K of T1 + 1.4K of pop + some culture (probably about 1K). I don't think you can do it in O&G, not by a long shot. I don't even account for AWKPs and coin/supply requirements for manufactory (granted, pretty minimal).
The main issues though: my T1 manufactory is
required in order to advance through the story quests. I have 0 choice in this matter, I
must keep one or two of them at a minimum, or else I forfeit the rewards of the main story line quests. (which will include diamonds - the ultimate prize for freemium players!
)
So for me, and for I dare say the vast majority of players, the need to keep space to run at least a couple manufactories is a moot point.
What makes a good event grand prize IMHO is;
a) Does this do something the rest of my city can't do very well and/or at all? (ie: Phoenix's atk boost)
b) Does this do something much more efficiently than X/Y/Z in my city does? (ie: Carnival set is stupid-good T3+mana+seeds production)
c) Is it worth the effort required to obtain it?
Unfortunately, while this new building can kind of slot into 'b', it ignores option 'a' entirely, and really seems to fall short of point 'c'.
So sure, I *could* replace 1 of my T1's with this, but really, considering the effort involved and that it's only about +1k goods vs. what the 1 factory is already making @ just Lv16, it seems like a helluva lot of effort for very minimal gain...
Now if it gave say, T2, which seems to be harder to keep stockpiled, and/or did something besides just T1 and a token kp instant, it might seem more attractive.