Toonz, that's why we're discussing what is really obsolete and what isn't. If engine X is no larger, no more expensive, and at least as effective as engine Y, is there ever a reason to use engine Y?
To be more specific: The Terrans start off with the techs that give them the ability to make the base "Impulse Drive," "HyperDrive" and "Ion Drive." All of them produce the same amount of velocity. The HyperDrive costs twice what a base Impulse Drive does, and is 50% larger to boot. The Ion Drive costs 50% more than a base Impulse Drive, and is 25% larger. There just isn't a realistic time you'd ever want to use a HyperDrive or Ion Drive over a base Impulse Drive. This is why automatic obsolesence (or any obsolesence for that matter, you can obsolete ships, but you can't obsolete components, as near as I can tell) would be beneficial.
In my current game, I've researched all drive techs, so I have 14 drives available, enough that I don't always remember what the best drive for any given purpose is. Now, the most compact drive is the "Warp Drive Mk V", and there's also no drive that is cheaper, though many tie with it. So, until the numbers change, this drive would never be obsoleted by the mechanism being discussed here, precicely because there may be a time when you need the smallest possible drive.
Now, except for the "Warp Drive Mk V" there is no other drive that has any desireable characteristics compared to the "Hyper Warp Drive III". Nothing is cheaper to build, only the WDMV is smaller, and nothing else is as fast (by any measure, since the WDMV is half the size but a third the speed). So even if I'm looking at throwing out fodder ships, I'm still going to use the HWD III, since nothing is cheaper.
Oooh... now there's an idea
Picture if you will, a tech tree similar to what we have now, but with a twist.
Each engine tech gives you three engines. The "minaturized" engine, designed to be as compact as possible, no expenses held back, the "cheapskate" engine, designed to be cheap at the cost of space, and a third engine that is a reasonable balance between the other two. Maybe even a third that is expensive, not as compact, but is actually faster. Then, the next two or three or however many techs in the propulsion line are "refinement" techs that improve those two or three engines in such a way that engine X without the refinement is never preferable to engine X with the refinement. Then, when you've finished the refinement techs, the next tech is the next type of drive, and once you've got that, you've got big and expensive engines that don't compare well to the refined engines of the previous level (will beat those engines in one category at most) and you start the refinement process over again.
This would add to the strategy of starship design, because even within a single generation of drives, you would never have a "use this engine, it beats all the rest" situation. You would have to decide on engine cost vs compactness in every case.
That's not what we have now, but I don't think it would be hard to change to something like this. The question is, what does it really bring to the game. I like what I think it will bring to the game, but that doesn't mean that I'm right.