Empty seat next to Plat pax was never a given, more of an unpublished perk that was actually a double-edge sword. While you would often end up with an empty seat, full cabin meant you'd often end up with your worst run-off-the-mill, last-minute neighbour.
Data point from 2h ago: 13h long haul in Y where the seat next to mine was empty until T-1h. My fellow PAX was obviously not a FF as he asked twice where he should sit; whether the paper he had in hand was a "boarding pass" or an "immigration form"; and whether there was a time difference between departure and destination cities.
I'd have no problem forking out real money for the guarantee of an empty adjacent seat in Y. Of course, that would have to be a firm offer, not a "empty seat unless we can monetise it more" offer, in which case it's basically moot.
Actually, even for short-haul, this offer would be perfect. When you are Plat, you already have lounge access and SkyP, so the J crackers/cold platter are the only real perk of flying J aside from the free adjacent seats. Make it a paid option, and you are basically offering short-haul J for a fraction of the price to Plat members
. I'm pretty sure we can do without the sucré/salé for 2h.