Expensive tech would only force player to tons and tons of missions before ANYTHING gets unlocked. Still he would research everything, since there are no time limits.
It all depends on implementation. As it is now, there's already a TON of often very weird sets of requirements, and the why and when is FAR from intuitive.
For example, already in the dead phase of the game I still find myself stuck in, one of the things I researched was the Rail Driver. I had one lying around in my stores for years, but because the intereconnected techweb wills it, my gals were unable to take it apart to tell what it was, how to use it, how to build ammo for it etc.
If the MAIN techs (A, B and C from my example) were easily accessible with requirements you almost cannot miss, but advanced techs would require a live captured high-ranked enemy, this alone would naturally push you into one path or the other depending on which enemies the RNG spits out in your game.
Have a lot of academy presence? Chances are you'll concentrate on energy weapons, as the best techs require academy provosts.
Many mercenaries? Heavy Guns will be your thing. And so on and so forth, every "faction" would provide access towards perfectioning research in a range of weaponry. But none of this perfercionist research would be mandatory.
In a game with balanced enemy presence, you might find yourself approaching the endgame without having explored either branch in-depth.
Thoroughness would bring it's rewards - many fancy toys to play around with - but none of it would be strictly neccessary, so if the chance to capture high ranking individuals never presents itself - no biggie, none of that stuff is strictly neccessary, not having any of it won't stop your game in it's tracks.
If you still want to linger around obsessively capturing every piece of tech, sure, just don't research the gateway tech and wait for the RNG to bring in another faction.
Does this really sound so bad?
Making lots of tech unmandatory would lead to geometrically-expanding list of unresearched (and never-to-be-researched) topics.
You're contradicting yourself. First you write: If techs were made expensive, players would STILL research everything no matter how long it takes. Because if players are able to, they'll research EVERYTHING.
But then you write: If not everything is strictly mandatory, players will leave stuff unresearched anyway. Because if players are able to, they'll skip EVERYTHING.
Now what?
I think you're right on both accounts, but for different kinds of players.
The mod as it is now FORCES all players to research most everything in each and every game.
And screw those whom the RNG gods don't bless and who find themselves unable to research some of the exotic thingies. (i.e., me in this game).
Sure multiple paths will be nice, they're planned, but fuck son, do you think it's that easy? I'm alone on this job 
Did anybody write anything that was proposed has to be finished next week?
If none of it ever makes it into PirateZ, so be it - we're merely having a discussion of things that MAY be worth looking into.
And you're the one making the calls.
I get that most games of this kind have the kind of tech tree you're proposing, rezaf, but this is IMO one more reason to do it in a different way (more akin to civ games).
Yah, I realize that. I can only repeat myself, though - I think PirateZ has far too much content in it to force players into researching EVERY LAST BIT OF IT in each and every game.
It also harms replayability, imo.
