I mostly agree with you, but...you've cleared out the jungle (which most junglers don't even fully do, because Krugs are way over in the Twisted goddamn Treeline and nobody has time for a goddamn Odyssey to get to them followed by a Trojan War to clear then). You can now
- Gank
- Countergank
- Get some deep wards down to protect your team
- Counterjungle
- Look at objectives (Dragon and Rift Herald, but also getting damage on undefended towers, clearing skuttle crabs, clearing out plants, etc)
- Try to clear some enemy wards down with plants, Red wards, or sweeper trinket.
- Coordinate with a laner to try to harass or kill the enemy jungler spotted with deep wards. Not quite the same as counterjungle, not quite the same as counterganking.
- Go back and get items, refill potion, buy more red wards. Lower levels don't back nearly enough, but it's usually better than trying to force a gank if they aren't overextended and you have high gold.
The problem isn't that there isn't an opportunity cost for ganking, the problem is that the reward for ganking is too high. Usually a summoner + a kill if it's well executed (which is easier than ever with how hard it is to be tanky and how much burst the game has now), heavy tower damage with the threat of first tower if the jungler doesn't answer or the laner doesn't tp (which can often be punished with a second gank) and a huge gold and xp advantage along with wave control if the wave is managed properly, which can just completely shut a solo laner out of the game. Ganking is too easy and consistent, and farming is too painful and not rewarding enough.
And I say this as a long time jungle main (though less so than I used to be).