It's never 'too late' to address an imbalance in gameplay. A lot of times, imbalances don't even make themselves known until far along down the road - for example, Rail Guns were an extremely powerful unit for over a year and it took until over halfway through Arctic Future to fully realize just how powerful they were for them to be toned down a bit. On that note, Hover Tanks are now somewhat in that position and could perhaps use some tweaking to take them out of prominence as well. From a design perspective, it doesn't really make sense when military units stay relevant and effective for 2 whole ages after they've been researched. You could say that it's because Inno didn't make new units strong enough, but it's also a lot easier to tweak one extreme example than it is to tweak everything else around it to compensate.
Anyway, that got off-topic. Regardless of what the Arc is theoretically nerfed down to (and let's be honest, it's probably not going to happen), by having one you're still getting larger GB rewards. Doesn't matter if it's 90%, 50% or even 10%. That's an extra % boost you don't have without it. Just seems to be like it's pretty excessive compared to other GBs. Military GBs scale much slower per level, the Orangery and Seed Vault also have very low increases, and the Kraken is one other that reaches a high percentage but can only work x number of times per day. Imagine if Zeus (and the two others), Orangery, or the Seed Vault had a 90% effect at level 80 like the Arc does. Think that might be a little more game-changing or even unbalanced? These things would be to our benefit, but regarding balance I think those are bad moves just like I do with the Arc's current design. Some of the attitudes expressed here are another problem that's pretty closely tied to the Arc: Greed. The Arc enables so much walking all over players who don't have one or are in FP swap groups that it's not even funny, and these players revel in essentially robbing players with very little FP/FP generation of rewards in order to add even the smallest amounts to their ever-growing FP wallets.
This is where I start soapboxing, but a large part of me is advocating for an Arc nerf just to remove that potential from the game. Sniping has always been a thing even long before the Arc, but it's gotten so much worse since its introduction and this sort of thing has caused so much drama even among guildmates that something should be done about it just because of the social impact alone. It's on par with plundering to sweep the rug out from under someone else using your huge Arc bonus, in my opinion. And then to add in 'well they should have locked in their spot then' as an attempt to justify your actions really speaks to how selfish a lot of people really are.
@Lady Egwene
It's not so much that the rewards are too high, though that was also addressed by adjusting the cost to level the GB. The main problem is that people were tearing the GB down and rebuilding it multiple times in order to continually reap those higher rewards. That behavior falls pretty square within 'exploit' territory to me.
@ITown
If anything, your write-up indicates that when something's unbalanced for long enough, it inevitably gets nerfed. Also, the Arc (or more accurately, Future Era) was in the game for about 3 months before the change to GB/ranking points so it definitely wasn't introduced to make up for lost ranking points. But as I outlined in one of my replies above, sometimes it's not immediately apparent that something is ultimately too powerful and needs to be toned down. I think the Arc is another example of that and I personally hope that it's only a matter of time before it receives the rebalancing it deserves, like so many other unbalanced things before it.