You know, this is why I liked old AI War more.
The more changes keep coming the less and less viable certain tactics were and the more and more there becomes only One True Tactic.
I do not like sandbox games with One True Way.
Well, harder games tend to be less "sandboxy", as harder games tend to reduce the effectiveness of most strategies.
One goal of 5.0 was intended to help "correct" some of the unintentional easiness of 3.0. They said that the 3.0 was a low point in difficulty, and that was not intentional.
Sadly, this means many people who grew to love some of the suboptimal but still very fun tactics that you could do in 3.0 all of a sudden are finding this not working very well at all in 5.0. In other words, lots of people (including you apparently) were spoiled by that.
I disagree with your implication that there are very few viable tactics now; there are still plenty of ways to win the game. But I will admit that many tactics that worked decently in 3.0 no longer work very well in 5.0
If those types of tactics and strategies that worked in the 3.0 world is what you got you to love the game, then that's fine. No one is stopping you from playing that version. No (reasonable) person would look down upon you for doing so as well. Just keep in mind that despite how long the 3.0 era lasted, it was the exception, not the rule, in terms of pacing and difficulty.
If you want, you can try 6.0 difficulty, and maybe you will get a similar experience that you had before. Difficulty 6.0 now is more like what difficulty 7.0 was in the 3.0 versions, due to many factors (especially the AI getting smarter and the removal of many of the caps they used to have)
EDIT: Ninja'd by Keith, who managed to make some of my points much more understandably than I did.