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.