This is false. Spammers can and do test their own emails against spam filters. And the less transparent ones, like Gmail's often suffer longer failures to detect new strategies spammers are using.

There are enough unique filters that a spammer cannot hope to account for them all in a timely manner. Getting .001% of spam emails past Gmail's filters is much more efficient than getting 100% of emails past my personal Thunderbird spam filter.

That's a valid argument for not using Gmail, but that in the case of Youtube, almost everyone wants a single place where they can watch all the user-generated videos.

> almost everyone wants a single place where they can watch all the user-generated videos

I have my doubts this is true, so much as people are used to the relatively common controls/experience that comes with clicking a YouTube link. If I gave you a link to ocdtrekkietube.com/s8fsj2 (not a real link), and it worked as well as YouTube, I'm not sure any user would be particularly upset about it.

The biggest reason YouTube is as powerful as it is is that video hosting is expensive and few companies can eat that kind of bandwidth and storage without being an ad giant.

We're talking about Youtube's recommendation algorithms here, not their video hosting. Imagine if Youtube hosted the same videos, but provided no homepage, recommendations, or search engine. People would still gravitate towards a single source where they can watch all the content they want. There may be some more competition, such as a liberal recommendation engine, a conservative one, and one for teenagers. However, these websites/apps would likely face all the same problems that Youtube does today.