It's a Catch-22 with how they want the game to progress. With little sustain, people are less willing to try to trade because they don't want to lose, causing them to be out quite a bit due to lack of sustaining mechanics. However, with too much sustain, people don't want to trade because there won't be a point to it if they can heal through it or just recuperate out of it quickly.
Champions with high innate sustain seem to be disliked by Riot because they could encourage passive gameplay by their lane opponents. They are trying to get people to be more aggressive, but high innate sustain makes that more difficult to deal with.
The fact that their goals in the jungle were not met so far is an interesting problem, really. The fact that the enemy jungler will want to gank you more often because the jungle isn't doing well for them, it encourages passive gameplay without high sustain. The only way to get said sustain now is through life-steal, which was nerfed, flask, which was nerfed, or pots, which can cost a lot if you rely on them for your sustain. Champions with innate sustain used to be a great choice for top because they could effectively win trades, allowing them to be more aggressive with less worry about ganks from the jungle, but it seems they didn't like that.
Really, they should focus less on sustain right now and look more into making the jungle better for the jungler in the early game, thus reducing the necessity for the early ganks. When that's done, they can begin to look at high-sustain champs to see if there is something that needs to be done. Doing it backwards from this order is just going to make everything more passive in the long-run, not more aggressive.