You also have to account for it being easier to get strange chests, not just through the coli.
When I joined, you could trade with swipp for strange chests, but you had to hope that you were on at the time the items you got through the coli were in the trade, and most of the items (from what I remember) for each trade did NOT drop alongside the other item (there were a couple that did).
With the Grand Exchange, you don't have to rely on RNG and luck to trade. They are available all the time. So, you aren't going to miss that trade with the items you have, because you were asleep or at work.
You now also get item drops from just about any activity, which, while isn't as good as the coli, still helps. Strange Chests themselves drop from most activities.
Re: to the people tanking the chest prices
You also have to account for it being easier to get strange chests, not just through the coli.
When I joined, you could trade with swipp for strange chests, but you had to hope that you were on at the time the items you got through the coli were in the trade, and most of the items (from what I remember) for each trade did NOT drop alongside the other item (there were a couple that did).
With the Grand Exchange, you don't have to rely on RNG and luck to trade. They are available all the time. So, you aren't going to miss that trade with the items you have, because you were asleep or at work.
You now also get item drops from just about any activity, which, while isn't as good as the coli, still helps. Strange Chests themselves drop from most activities.
This again leads to more supply.