A few issues make fold-up wing tips unfeasible:
Weight and complexity of a mechanism and control system for it.
Shifting CP forward makes the airplane less stable statically. In any case, if you want to reduce trim drag, just move the C of G aft to unload the tail to get the same result, and use computers to artificially induce static margins, which is already done.
The stall speed reduction from the extra wing area is quite small. A wing tip that added 10% to wing area when dropped would only drop the stall by a few knots. The stall speed reduction from adding flaps dwarfs the reduction you get from adding area.
At high altitudes, you want the extra span so you wouldn't want to retract the tips anyway. On an airliner that cruises sub-sonic at high altitudes, there's no benefit to reducing span. On sub-sonic airplanes, where you clip off the wing tips, it's to improve speeds at low altitudes (like the clip wing Spitfire, done to improve performance below 20000 ft - for the very high altitude versions used to take on JU-86s, they added span).
The only practical reason to have folding wing tips on an airliner is to reduce span on the ground, as an alternative to winglets (which are used to gain some of the benefit of added span without taking up space or adding to bending loads on the wing), but a winglet is a much cheaper solution.
Which, as always, is why you haven't seen anybody do folding wing tips in the last 60 years, whereas winglets are everywhere.