Hm, I can't say I agree. S7 specifically had some very heavy scifi episodes: Emergence
dealt in the creation of new life, Attached
explored merging consciousnesses (while also being one of TNG's strongest character episodes), Interface
considered telepresence, Homeward
is classic social scifi, heck even Masks
is pretty damn high-concept. If anything, TNG's scifi-heavy episodes got a lot more sophisticated and smarter as time went on, IMHO.