What is the point of wiping the waystone map if we die to the boss?

To waste our time? Piss us off? What game design purpose is there in kicking a player in the balls while they're down just because they happened upon the boss first while clearing a waystone map? There isn't any, it's bad design.

This is the 2nd time now I've lost an entire map because of the idiotic game design. You all seem very keen on introducing things into the game to make players miserable for the sole purpose of making them miserable.

I thought about maybe spending something on MTX to help "support the game" but if this is how GGG treats their players? Hell no. I'm already regretting the $30 just for EA to a "free" (except not free at all because if you want to engage in the market economy or have even a sliver of stash tab QoL that games like Diablo 3 did a better job of providing, you have to spend $ to fix invented problems) game.

What is the point of posting in Feedback section and give no feedback at all?

Answer to your question is get better at the game.
Also, you can start with the boss if you find him.