A wide strip of gravel on the left side of the track after the second turn in the chicane would be enough to prevent drivers from rejoining at full speed.
I don't think the whole run off area should be gravel if only to give drivers some space for a non-fatal mistake when trying to overtake in that spot.
Well that's in danger of taking us away from the topic, but I strongly disagree. Prior to the introduction of that small car park on the site of the former gravel trap at the last chicane, Rosberg couldn't have made that error and kept his position. That, in actual fact, is why the previosuly infamous "wall of champions" now claims so few victims, because it used to be that if you outbraked yourself on the way in you would miss the first apex, take too much kerb on the second apex, and that would throw you into the wall. Now if you outbrake yourself you go straight on. You can't criticise Rosberg for it, but when the layout was more challenging, he couldn't have made that error and gained from it. If he aborted the corner early enough he'd have had to go over the gravel and given himself front wing damage and/or two airboxes full of shingle at the very least. And if he tried to make it and hit the kerb, he'd have been in the wall.
But we have what we have and it would help if people were clearer about what the rules are. There are two possible views on what a driver has to do in those circumstances to ensure he gains no lasting advantage. If you only look at it with respect to the car immediately behind, since the following car had more pace, it was sort of inevitable that Rosberg would gain no lasting advantage in that, by cutting the chicane and pulling away slightly, it was only a matter of a very short period of time before Hamilton caught him again. Is the mere fact that Hamilton quickly caught Rosberg enough to make the stewards conclude there was no lasting advantage?
I think the above is a valid interpretation, but Rosberg seems to think not because he says he realised he had to give back the time he'd gained, which he says he did almost immediately, in turns 1&2. But his laptime on the lap where he cut the chicane was much quicker than his previous laps and that was quite clearly as a result of the shortcut. And his laptime on the lap after was not sufficiently slower to suggest that he gave back all of the time he had gained. Maybe he did and was then able to pick up his pace in the remainder of that lap. If so, he slowed down again a lap later.
In my view the sector times need to be looked at, and rather like under a yellow flag, you should have to promptly offset any apparent time gain by posting slow sectors relative to the sector times you've been setting. And drivers should be expected to lose enough time to ensure that there can be no room for reasonable doubt that they gave back the time they gained through leaving the track. I've no problem with drivers waiting until they have an opportunity to lose that time without exposing themselves to the car behind and losing position, but any time gained relative to absolutely any other driver anywhere on the circuit, ought to be given back and be seen to have been given back, promptly. Because, as happened yesterday and can happen in any race, the time gap between cars that don't appear to be in the same race, can become crucial. If Rosberg gained time against Hamilton it was of little consequence because Hamilton was able to just catch him up. But he will have also gained time against Perez, Ricciardo, Vettel, Massa etc. You wouldn't have thought at the time of the incident that Rosberg's gap to those cars was of any relevance, but as it turned out he spent most of the remainder of the race trying to defend and manage that gap and to stay ahead of those cars. That's my interpretation, but it should be noted that my interpretation is consistently harsher than the stewards'. So I don't think the decision in this case should surprise anybody, even if they are still upset about an incident that happened six seasons ago which isn't even comparable and which occurred when the phrase "lasting advantage" wasn't even in the regulations.
The stewards obviously don't agree with that and judged that no advantage was gained, but still warned Rosberg about track limits. Personally I'm in favour of that if it's done consistently, which it isn't (I don't think Kvyat was warned, was he?). It suggests that the stewards take seriously the rule that says you can't leave the track on purpose, and it gives you the impression that they're saying "Okay, you can do that once if it's by mistake and there's no advantage, but you can't keep doing it. You're supposed to be one of the world's top racing drivers and we, the stewards, aren't going to buy it if you make the same "mistake" again and again and again". Warnings before penalties for accidental off-track excursions without lasting advantage are a good idea in my opinion.