I almost never want previews. I avoid programs that automatically download stuff without an explicit request, partly because this sort of functionality is pushed by companies that make their money out of tracking me.
An ignorable popup to add a preview might be okay (if it couldn’t be hit by accident), but if I just keep typing I really don’t want it to be fetching web content. Apart from anything else, there will be false positives on link detection. I want to know when what I type is public, especially in an encrypted app (where there is an expectation of privacy). Any app which might suddenly leak part of my message if I mistype or forget there are previews is not safe.
Suppose I manually share an invite link, for instance, maybe not a DC one. I really don’t want a preview!
Previews defaulting on in unencrypted chats and off in encrypted chats might be an option, but a bot (which can be kicked out by all parties) seems a better opt-in, since it requires every participant’s consent.