Is Mapbox moving away from Leaflet as they develop Mapbox GL?
It's true that Mapbox currently focuses its efforts on GL, but they are not "moving away from Leaflet" — Leaflet will continue to be supported as one of the first-class clients to be used with Mapbox services (and GL-based styles have a raster tiles fallback). Leaflet is very mature, has a huge community, is suited for most simple mapping needs, is very easy to learn, and doesn't require a lot of development effort from the Mapbox side to support.
Mapbox GL on the other hand represents a vector rendering technology, which is very different. It is much more powerful than conventional raster tile based clients, and enables applications never seen before in the online mapping industry, but it's also many times heavier, more complex, and much harder to learn. Rendering vector data on the client is something Mapbox bets on as the future of mapping, but it's incredibly challenging to build, so it requires a lot of focus.
I'll continue to maintain both libraries — while they have some overlap, there's a place for both.
It came to me, that after all, they have to, if they want to display ortophoto or satellite tiles.
I was thinking more in the terms of street maps or thematic maps, where mapbox-gl-js displays them in a very nice way using vector tiles.
I wouldn't say it's more complex. mapbox-gl favors a more declarative style (look into "Map Style" in the docs), but the concepts are not that different than Leaflet.
Web GL features (such as video support) is an obvious advantage, and part of why we chose it. We don't use plugins for this project, so can't say much about that.