Engine revisions are done to make it easier to work with and to provide new tools to developers

I'd say it's done more in favor for new tools and systems for developers to use, not necessarily ease of use, but that depends on the purpose of the engine. The reason why Unity and Unreal are much easier engines for developers to use is because of their widespread use. They're a large or primary source of income for their given companies, so getting more developers using their engine is beneficial. And what better way to do that than making it easy to use. Just because Source was a headache for me to work with doesn't mean it's a poor or dated engine, it's a very powerful engine, it's just that the learning curve is steep and graphical interfaces aren't quite there nor do they need to be. Source isn't some widely used engine like Unreal and Unity so making it easier to use for people outside of Valve isn't a high priority. And for instances like the 'Valve Workshop' or 'Portal Map Maker', they just build the tools for the specific systems that are needed to make those aspects much easier to use.

I can't really comment on the optimization on the mobile platform. But I do know it is running on Source and that they are reusing a significant number of assets from Dota. So if you look think it looks dated, chances are that's because you're looking at some assets that were created as far back as 2011.

It makes sense why they haven't released major updates over the years, there hasn't been any need to. They don't make games.

They do make games, they just don't release them. :)