Yeah it's a good guide. Unfortunately Uber started it about a month after I took a year and a half break, and I pushed too far into the CE map to get the TE units later. Trying to figure out the right path by following the questlines on the Fandom wiki was a real pain, and there were places where I missed units. But I did manage to get all the FE and AF units, and I'm working my way through the OF map now. Inno really dragged out the AF and OF maps IMO. It's a slog. But I remember how they released both ages slowly and in several parts too. Having the units several ages early is definitely worth it though.
I think Inno screwed up by leaving things open like this. Given the convoluted nature of some of the parts of this guide, I don't think it was intentional. But I think the story questline really should have had tech blockers in every age so that people couldn't push ahead like this, that is to advance a player would need to research a tech from the current story age. A player using very advanced units to attack neighbors in Indy is the sort of thing that might make players get frustrated and quit. I'm not sure how much people are bothering with PvP anymore though, my cities on both J and P haven't been attacked in months. I might have been attacked on A and/or G but they're semi-abandoned WW farms, so who cares?
They've had evolving philosophy over time. I think some of it was intentional, but with poor foresight. Which it's far from the only feature that the reason it exists as it does currently had to do with that! (looking at you Arc, siege camps, etc).
In the first ages there was tech blockers in almost every age - not even necessarily to be a tech blocker, but to encourage you to maintain a pace through tech that map wouldn't be too frustrating.
Indy and PE story were pretty much intentionally designed to be possible in the age prior to them - quest conditions have explicit options to avoid being in the current age.
ME & PME they similarly made quite flexible to get ahead, removing references to specific ages, and replacing them with "current age" until the blocker at the end of PME story. So I think that far was quite intentional.
Bear in mind at the time the uses for advanced units were also much fewer. You could use them to keep fighting more on continent (fair enough), or to do some amount of fighting in GvG in ages you hadn't yet reached - but since ages were agelocked in terms of troops used, you weren't getting any unfair advantage. And then there was the neighborhood, but hoods also routinely spanned 3+ ages of players, it wasn't unheard of to have your spears getting stomped by tanks anyways. So fairness was not a concern there either at the time.
The PME "Truth or Dare" oddity would've also been of no consequence at the time they added it as there was no story to skip *to*, and the model they'd been using to that point didn't skip as far as i'm aware (every story quest having a prerequisite of other story quests).
CE is probably where they lost control, but noone noticed for a long time. They started having quests that had prerequisites other than "did the previous story quest". And they missed some of the linkups such that it was possible for story to disappear. This required that there be a way to get story *back* so that your game isn't bricked, without having done some of the story before it and enabled the leap from pre-PME as well as those are the points we skip *to*.
For a while the consequences were relatively low still and they made no effort to stop people from fighting ahead of them in TE/FE/AF at all. And it wasn't until OF part 4 that someone decided things had gone too far, and they needed to put a stop to this. At the time the tech blockers were still mostly considered unavoidable. So you had CE players with what this guide shows you how to get in Industrial - and I think that was intentionally possible as well.
The consequences to this point were still relatively low. You could have easy GE and a strong city defense but is that so bad for the work involved? Regardless internally they obviously decided they didn't want that happening any further, because it was never again a possibility - not even 1 age ahead. And they probably hoped that'd be enough, because a core assumption of their game design is people will advance and move past the deficiencies of previous eras - that they usually will not have to go back and "fix" old content.
Then came GBG and impossibly crazy amounts of boost on defending armies to overcome that "normal" methods would struggle to counter. Shortly after its release they bumped the attrition cap even higher, and I assumed it was because Hover Tanks in CE might've been "too strong". I even made the suggestion at the time that if that was the problem they should just age-lock GBG like they do GvG instead to keep attrition cap more approachable by normal means. Their response though was that they didn't want GBG to be something where you fight all day and it was important that the cap not be approachable. Basically they were building in room for power creep as well so that thinkable amounts of attack boost now would still be challenged down the road.
Sometime after that the discovery of how to pass the PME and CE blockers became public and brought us towards the current situation where OF units in industrial and FE units in progressive have so much of an advantage that they can cope with that ridiculous attrition cap. However since the "meta" in GBG involves free fights and swaps instead of competition between guilds, it's hardly its biggest problem (the world where I'm focused on GBG with advanced units I use it to bring battle *back* to GBG - and its not without costs, but it lets me find some fun in a currently very broken feature).
I still would not be surprised if when they get off their yellow bottoms and make more comprehensive changes to GBG than just the siege nerf they have on beta, that advanced troops may also become ineligible for GBG. And that's fair. If they fix GBG properly, taking away my ability to use turturrets in industrial there should probably be part of it (or if not taking it away, dissuading in some manner).