And the security of two chains does not add up, in fact, the more resources are split up between multiple chains, the less secure they all become. So this doesn't solve for the security side of the trilemma.
Also, as a business owner, you can charge an additional fee when accepting unknown or low volume coins, or coins that are backed by blockchains which have a small number of nodes.
Imagine how many more people would be able to afford to buy your products if you would accept just about any token as payment. It could be a way to gain a competitive edge and boost your profit margins by accepting riskier coins from unfamiliar communities.
It's not quite the same as if everyone could print their own money on pieces of paper; we know that wouldn't work because nobody would have an incentive to do any work to produce anything... But the opposite extreme (which is our current reality) also doesn't work; there are all these people with no money and no opportunities who could be doing business with each other, but they can't because they have no money and are not allowed to create their own money to get their parallel 'unbanked people' economy going.