Earlier quoted context omitted.
Besides, capturing text from other applications is very much required for various utilities. It's as much of a security feature in Wayland as turning off your computer and never turning it back on is.
There is a separate, privileged, interface that this kind of utility can use. Meanwhile, the other 99% of applications don't need unlimited permissions.
You also underestimate how many programs make use of functionality that could be abused in some way. And unless you lock all those interfaces down it's all security theater. Who cares if the display protocol disallows copy paste snooping when there are a million different ways to get the the memory of other processes or the files that they store sensitive information in. And such a locked down ecosystem is antithetical to free and open computing.
I don't use my computer to be secure, I use it to get shit done and and to have fun. I'm not going to accept approaches to security that interfere with that any more than I will accept the same in real life. There aren't any bars over my windows because we have functioning police to deter criminals. I don't need lab tests done for all the food I buy because we have regulations that ensure food sold is generally safe to eat. I go outside without body armor and weapons even though someone could theoretically kill me. 100% security is always a tradeoff for quality of life.