Youtube downloaders => Install them on Chrome dev channel, canary or switch to Linux/Mac/ChromeOS
If you still want them on Windows stable, turn on developer mode in chrome://extensions
And AdBlock is very much alive on Webstore: https://chrome.google.com/webstore/detail/adblock/gighmmpiobklfepjocnamgkkbiglidom
It's obvious that Google is doing this to prevent crapware (installed by many naive Windows users) from hijacking your browser. If you're not a naive Windows user, why not move to dev channel?