I think I may be in the minority...
I read most of the comments in this thread (I admit I skimmed in a few places), and I think there are a lot of interesting thoughts and valid arguments. My following thoughts synthesize several of these ideas, but it would be a nightmare to try to quote each of you whose thoughts may be in some way presented below. So if you think, "I already said that," or "That was my idea," that's fine--I'm in no way claiming these thoughts are entirely separate from what has already been posted.
First, I think this issue has to be looked at as a game-mechanic issue and not a realism issue because the very premise that a sprawling galactic empire can only research one technology at a time is by nature unrealistic. There are other factors that make the way GC handles technology inherently unrealistic--but the game wouldn't be fun if its major features (population/culture, economy, war, research, etc.) were not abstracted.
So while I appreciate a certain level of realism, I think the foremost consideration for the tech tree should be how it actually plays. Is it simple enough? Does it allow for strategic choices? Is it balanced with other game factors? Is it fun?
The GCII research system--which has been described in this thread as "free" because of how it allows players to beeline to their advanced tech of choice--worked fairly well in GCII, in my opinion. It was simple to use. It allowed for strategic choices. I also think it was fairly well-balanced. And it was fun.
But the GCIII tech tree is quite a bit different from the GCII tech tree. In GCII, there were several techs that didn't do anything other than unlock a future tech. In GCIII, however, every tech (to my knowledge) has some effect on the game--providing a bonus, reducing a cost, allowing a new component, etc. This key difference causes me to question the viability of a "free" system.
A "free" system is easy to use, but might be difficult for new players to learn (i.e. too little direction on how far to research a particular tech branch).
The "tech age" system is simple and easy to learn (harder for new players to make really bad tech choices).
Because of how much more the GCIII tech tree has to offer (which I love), I think that creating a list of cross-requirements for each tech would be too complicated and difficult to learn. I think the "tech age" system is a fair abstraction of a cross-requirement system.
I think a "free" system offers the highest number of potential strategies--but many of them are actually bad strategies. I think providing some structure helps players (especially new players) to find workable strategies that they can implement in different ways, at different times, and with varying levels of ability/success. Limiting players with the "tech age" system, in my opinion, helps raise the bar and promote feasible strategies in a user-friendly, balanced manner.
As has been said, you can still execute a "beeline to planetary invasion" strategy--but the path is just different than in GCII. The current "tech age" system doesn't prevent this strategy, it merely changes the means of implementation.
I'm not sure how well a "free" system like GCII's would balance with the number of techs and amount of effect each has on the game in GCIII.
Creating cross-requirements would be a method for creating balance, but as said before, this could be difficult to learn and become no fun. As a viable alternative, the "tech age" system functions in much the same way as a cross-requirement system and helps prevent unbalanced playing.
A "free" tech system was fun. But I don't have any problem having just as much fun with the "tech age" system currently in place.
All that being said, I'm not saying there aren't other simple ways of limiting players from going too far down a particular tech branch. I think balancing the tech costs to make advanced techs difficult early on is one way of limiting players. I also liked the idea of influencing the cost of each tech by the ratio of its cost versus the total number of research points spent.
But I think the "tech age" system works fine and is fun, and without any strong reason, I don't see a need to change it.
I'll reserve further judgment for when all the techs are playable.