[Suicide Junkie]

In addition, consider personality tech paths - an aggressor would find great torpedoes, missiles, beams a must, but a more defensive player would attain ECM, advanced shields and armor. A simple choice between PDC and Missiles, in the beginning, would make later techs (Advanced PDC or Advanced Missiles) obsolete, accordingly.

I fail to see why that should prevent you from researching the other.
And besides that, how could an "Advanced" Version become obsolete when you're still using the standard Version, unless you find an "Uber" Version of the same tech?

A change of leadership, or pressure from another race could all make you decide to develop the techs you neglected before.

"Hello, Sir. I know I'm just a historian, but back in 2402.5, there was a big question of whether to work on Tech A or Tech B. Since our highly developed tech A is useless against these aliens, I suggest we try Tech B instead. If if proves effective we could continue to develop it to its full potential and have these nasties on the run!"