Why not?
You seem to be referencing a hearthstone deck building rule book that doesn't actually exist. It is the same logic that stops hunters running flare to deal with secret paladin or people running kezan to beat freeze mage.
Yes I can and do run cards like that to win match ups. It is fun!
Well, correct me if I'm wrong but from the start you've been going on about how "non-viable" the whole murloc paladin OTK deck is, how it doesn't do anything better than a standard midrange, is easy to counter etc, and your choice of a "easy counter" card is...counterspell.
By that logic I suppose we can say that Freeze Mage is a non-viable deck because not only does it auto-lose to CW 90% of the time, it is hard-countered by Kezan/Flare and therefore is too gimmicky. But of course we don't say that. Counterspell to Anyfin is not what Kezan is to Freeze Mage, but you are trying to sell it as such, which boggles my mind.
Same goes for FoN/SR. "Psh, counterspell hard-counters force of nature and therefore breaks the combo, its not OP at all guys".
This isn't Yugioh or Magic, where you get to choose when to activate your trap/quick cast spell during your opponent's turn. Even in the idea scenario where you've somehow managed to play counterspell on the exact turn before the paladin plays Anyfin AND he somehow has no other spells to trigger it first, unless he is dead on the following turn and therefore has to pull off Anyfin, he can just play his other cards and wait to draw into a spell. Specialized Anyfin decks run plenty of draw cards and a good number of spells, those that just tech it in obviously do not rely on it as their deck's win-condition.
No, I'm not saying that Anyfin is anywhere close to as strong and reliable and FoN/SR, but your example of it being "easily countered" just doesn't cut it.
And no, I'm not referencing some deckbuilding rulebook, it is simply how deckbuilding works for the competitive hearthstone decks. You can't, for example, throw in 15 7+ cost legendaries into your deck and expect it to curve well because you happen to have one pair each for 1-6 cost cards. Sure, you can go ahead and throw in 2 counterspells into a tempo mage deck with mad scientists and a pair of Mirror Entities, but if you somehow expect the deck to function as consistently as the competitive lists being used by pros/on ladder then you will be very disappointed.
