Is curl|bash insecure? YES
Is curl|bash more insecure than most of the ways people install software they just download from the Internet? NO
So it's not ideal, but most people don't do anything else that's much better, and it's convenient, so you can expect it to continue.
Installing software outside the package manager is the exception here, not the norm.
Essentially, if someone wants to hack your system they will, whether it's patching your routers firmware, etc.
If Iran can't keep stuxnet out of airgapped systems, you're not going to keep any serious actors out of your systems connected to the internet.
We educate users to download software from distributions rather than trusting random sources (that might be very safe or not).
A recent example to add to this, for those who think "nobody who's not a complete beginner would never do that":
https://www.reddit.com/r/netsec/comments/3lefc6/chinese_ios_...
Is curl|bash insecure? YES
Is curl|bash more insecure than most of the ways people install software they just download from the Internet? NO
So it's not ideal, but most people don't do anything else that's much better, and it's convenient, so you can expect it to continue.