1. the auto-play does eventually stop, presumably because they need to maintain some semblance of legitimacy to advertisers

2. caveat emptor. how youtube works and how ads are played is hardly secret information, so accusations that google is deceiving advertisers (not just providing a subpar product) is hard to make.

3. people have tried suing companies for this, but they were over securities fraud rather than deceiving ad buyers. No idea how successful they were though. it's worth noting that suing for securities fraud is easier because unlike with advertising, you have to declare risk factors, hence why every company's prospectus lists all potential list factors under the sun.

Intent is fundamental to all criminal cases

The intent of auto play on YouTube is not to defraud ad agencies

What is the full intention of enabling autoplay by default?

To keep the user engaged since "doing nothing" is the default action for people