They already did, a few years ago. I think it was 2015. Before that we had 9 slots and lots of people complaing. Now we have 18 and far less complaints / request for more slots.
I think the lower number of requests for more slots is largely due to the deck export feature, whhich was added in 2017. I personally do not see any reason to have more deck slots. I have a large collection of decks stored outside Hearthstone, and it’s easy for me to copy/paste a deckstring to rebuild a deck I want to revisit. I personally prefer that over having to wade through perhaps dozens of screens to find the deck I want to queue up.
So for me personally, I would not care if Blizzard decides to add more slots. I probably would not use them.
For Blizzard management, they should probably make this investment is that makes a sufficient number of customers happy, or invest the money elsewhere if that elsewhere is more important to a larger number of customers. So it is good that you made your voice heard.