You can configure uBlock origin to only let through the ads, but still block the other garbage. Or you can pay for youtube - or you can find something else.
> they are simply detecting user agents with ad-blockers
The browser's user agent string doesn't change based on plugins installed, or at all--unless end user changes it purposefully.
The ad-block detection is likely and usually a separate script that runs an interval check on the existence/visibility of ad-related DOM elements (which would be false when hidden/removed by a plugin or even the developer tools)
I hope this move will strengthen alternatives.