For certain operations, yes this is required on large-scale systems.
Some things like "most active user" can be determined relatively easily on an event-centric index but others like your example are much more taxing because there are too many distributed joins required.
While it may sound a pain to have to create an entity-centric index there are benefits which you may not have considered. For example, with each session holding a list of video IDs that were watched it would be possible to build a "people who watched X also watched Y" stylerecommendation system. I know of music streaming services that use elasticsearch in this way.