Just from what I have seen, the price point, (that is currently unknown btw and like someone else pointed out Overwatch on PC started at $40), can possibly be justified to me almost entirely by a complete single player/story mode. The base mechanics may be the same as PvP but you have to take into account writing, designs for characters and locations (because they said everything is getting updates and the model comparisons at playoverwatch are quite telling), the design and creation and constant testing of new features like the talent tree and missions and AI and all things like that. They basically had to develop what could maybe be an entire game on it's own, using a system they already have (which is common) but kept it connected with the PvP.
I see it sort of like, they made a story mode, alongside a bunch of various updates (the game is like nearly 4 years old just by now and 2 is nowhere near out) but they don't want to splinter the player base and especially not the Overwatch League, so "2" is largely The Story but also still has the PvP to keep that going without resets and other hitches.
On another note: I never bought Overwatch. I don't have people to play it with. I have played the game before, and enjoy it, but I don't want to buy a PvP game and never play it. But Overwatch 2 has a story, and I love the story of the world we already have. So I am much more interested in Overwatch 2, and I am probably not the only person. And then, if I or these others want to, we can play the PvP the same as everyone else without any problems.
They potentially could have kept it as an update to Overwatch 1, but it would basically have to be a paid DLC to justify even making it in the first place, and then people who wanted to play the story would need to buy the original game.
So instead it might have been easier to repackage it as OW 2 as a complete experience.
But this is just my theory on it, and whether or not any price is worth that is subjective and not for me to say.