While I understand the rest of your point, and see where you are coming from, I really do not understand this.
Blizzard, like any company, has a finite number of staff. And hence a finite number of hours their staff can work on games. So the simple truth is that when an engineer is assigned to work on one task, that engineer is NOT working on another task. Same for a UI designer, for a tester, etc etc. Deciding which task to work on and which to postpone or even never pick up at all, is called prioritizing.
The release schedule of expansions is pretty much fixed. That will always get enough priority to ensure they finish in time. The solo content, once announced, falls in the same category. All those are main features that cannot be jeopardized so they will get the resources needed.
Then the leftover resources are available for all other tasks. Fixing bugs. Making minor but interesting improvements. Designing and coding new brawls. Working on optional future work such as e.g. new game modes. And, indeed, making UI changes that make controlling the game easier, such as a button that relieves the player of having to manually squelch each game.
And before someone responds “Hey, Blizzard makes loads of money. Why not hire more staff so they can do it all?”. Yes, they could hire more staff. That results in a 100% sure increase in cost. Business want to make money. Business invest money for that. So they will try to hire people up to the spot where hiring more staff increases the cost more than the revenue.
If you want to convince Blizzard to implement this feature, you will have to convince them either that this is more important than their customer base than other things being worked on, or that this feature will be so popular that it will increase revenue by more than the cost of expanding staff.
I personally think that Hearthstone is far more likely to lose players because of gameplay bugs such as the examples below (all taken from Known Issues List 2019 - Updated Dec 12th - #6 by Aelathera) than it is to lose players because there is no autosquelch. But I may be wrong.
Examples of some actual gameplay bugs I would like to see fixed before Blizzard staff should even think about autosquelch:
- Twilight’s Call can resurrect minions that gained Deathrattle during the match.
- Attacking adjacent minions (Sweeping Strikes, Cave Hydra) will not trigger Lifesteal or Poisonous if the main target has Divine Shield.
- Vendetta will not count itself for the Mana cost reduction when held by a non-rogue class.
- Unseen Saboteur can cause Chameleos to remain stuck on the screen.
- Twilight’s Call checks for minion that had an active Deathrattle effect at the time of death.
- Waggle Pick does not reduce the cost of minion that was generated from a Deathrattle.
- Da Undatakah does not return spells if it gains the Deathrattle from Test Subject.