If you really want to remove sightstone...
Why don’t you just change support items so they give a better trinket with their quest?
The yellow trinket could become a “sightstone”, with 3 charges that refill upon visiting the shop (it could have 4 charges once you reach X gold).
The red trinket could have its range/radius of detection increased and last longer.
The blue trinket could reveal a bigger area, give vision on a slightly bigger area and have its cast range increased.
This way supports would still have vision control and they would not give actives up. Junglers/midlaners could still get red trinket to clear wards and supports would still be the vision control role without giving a whole item slot up. Having a buffed trinket after those quests would give them enough vision without having to buy a “must” item like Sightstone.