Monopolies are a barrier to entry. I don't think any of google's products prevent that. As we see in most technology over the last 20 years, the companies at the top can change fast.

A company that creates a better search engine, will easily take over search. Next gen of search will be better at providing more relevant / less spam

Youtube does have some network effect and high costs but not enough to stop anyone from competing and vc money evens out the costs. Next gen in video may offer more features, or for a completely different platform like VR

The only platform I do see them having a strong ability to block out potential competition would be mobile and Android.