Even with the tech tree fully upgraded, every part should have a use. This is the case with most parts because more powerful engines, solar panels, generators, etc are also heavier so the early parts continue to have uses on lighter vessels. Alternatively, xenon engines are extremely light and efficient but very low thrust and only work in vacuum. There's always a cost when choosing one part over another so everything continues to have a use somewhere.
There's currently no reason to use the low tier science parts once we unlock the more advanced parts that are lighter but do the same thing. These lighter parts need a cost for choosing them over the early parts. They could cost multiple rare resources when those are introduced, or even require a kerbal onboard to be used more than once. (Incentive to bring Kerbals is always a good thing)