You have some solid points about limited entrances to an assault point. But overall, I'm inclined to disagree. I'm a semi-competitive FPS player, with a heck of a lot of hours into foreign FPS games that never made it to the west, such as Special Force and PointBlank, and I find Overwatch to be one of the most difficult games at high-elo, coupled with SC2 and some rhythm games like IIDX or Osu.

First, CSGO-esque games require careful planning and communications, but the issue you mentioned for Overwatch are prevalent in these games as well. Even though there are 2 bombsites and multiple ways to get to them, the general positioning of defending/attacking teams are fixed and are very dependent on the meta. However, I'd argue the creativity and strategic variety to get your team closer to victory in these fixed positions is what makes these games 'non-casual'.

So back to Overwatch, while you say

the game is just the two teams headbashing into eachother until one team is wiped out.

I hardly find that as the case, as players are required to

1. actively monitor every other players ultimates and cooldowns
2. pay attention to vertical as well as horizontal movement of enemies and also care about invisible targets.
3. pay attention to spawn times of health packs

In a way, Overwatch players have a wider range of strategies to execute, compared to CSGO which is inherently limited by its land-bound movements coupled with a limited weapon variety of primary fire, secondary fire, and grenades.

And lastly, I would like to ask, what makes a game casual?