Chaining wasn't really an issue back then because
1) attack times were about 20h. When everyone's gaining 10% of their land per hit attack times can afford to be longer. Longer attack times = less provinces can be chained during a 48h war. If someone wanted to chain me back then, all that meant was my kingdom would assuredly end up with more acres and win the war.
2) Change to the old gains system and provinces will revert back to double the defense compared to their offense. Back then, everyone ran say 80 dpa/50 opa, now its reversed. Offensive power per kd would decrease drastically, decreasing the chaining possible, and more provinces will be able to have troops out and still be unbreakable. What fun is it to have all these provinces that if they hit you you can hit them right back. Trading acres sucks.
3) With the longer attack times, the amount of provinces a kd could possibly chain would be greatly reduced. With #2, provinces can no longer double tap. All this adds up to it not being strategically wise to ever chain someone, as you'll lose acres if the other kingdom does 3 rounds of max hits while you try to chain say 4 provinces.
Chaining wasn't a viable strategy back then, that's why it didn't happen. Newer players tend to think the game and strategies "evolved" to a superior strategy. the only thing that has changed is WHAT was the superior strategy.
Essentially, the whole concept of chaining is bad. The age is too long and no one likes playing a 200 acre province. Getting chained leads to players leaving the game. Fixing GBP isn't the answer, fixing the strategy of the game such that a kingdom wouldn't WANT to chain another province is. The only time people got chained was if there was a real vendetta and you did something really bad. It was fun being gangbanged because odds are you did something fun to earn it.
The sheer destruction of the chaining and offense and the fact that a smaller kingdom has zero shot at beating a larger kingdom is also what leads to all the fake wars. Fake wars mean people sit and do nothing for weeks at a time just trying to avoid a kingdom hitting them. This is boring and drives out players as well.
You could go from eliminating aid to trying to do something like messing with the trade balance, or only allowing aid to smaller provinces, etc. but all that becomes exploitable eventually. the aid elimination issue is one that need much more consideration.