Typically, Blizzard makes the stats increase enough so that by the new max level, the greens and blues you have gotten from leveling and doing some normal dungeons would be better than any gear from the previous expansion. In some occasions, if you had the absolute best gear from the previous expansion (e.g. Naxx gear in BC, Sunwell gear in Wotlk, etc), some of those might last you until max level, but you would probably have replaced them with dungeon blues. Maybe a legendary weapon might have lasted till the first raid
AMR also doesn’t usually focus on gearing optimisation during leveling, since you don’t really need to min/max that, because you will typically only spend a few hours at each level at most. Their optimisations usually do weird things if you try and use it for non-max level characters, since they focus their pre-processing work on max-level characters with appropriate gear.
So while you need to throw out a lot of stuff in the next expansion, it isn’t really that hard to figure out what to throw out. I typically just keep the highest ilvl gear (particular the gear with the most stamina) when I’m leveling to a new max level.
However, the expansion probably won’t be out for another 6 months or so, it would still be nice to see this feature improved, so we can free up more bag space.