The win rate of DK decks is irrelevant to the current discussion.
Given the decision to include Viper, which is arguably the strongest tech option due to its low cost of 3 mana, premium stat line, and the tradeable ability, it only follows that other tech options should also be considered.
Choosing a different course of action would result in diminishing the effectiveness of weapon-based decks/cards while allowing other combo-oriented decks/cards like Boomboss and Plague DK to excel. aka favoritism
I concur that incorporating Steamcleaner into the core set is sensible.