You sound like a color defender. Rather than looking at the game as a whole you are trying to defend it because its good for your favorite color/deck. The problem is that it is to versatile and to good. I’ve tried three different decks. Big Green, Red Combat Burn, and Yellow Sacrifice. Frog Tosser single handedly disrupted all three of them.
You have to be very careful with “comeback cards” because what happens if they are used when you already have a slight lead or are equal? They completely give you the advantage. Look at doomsday arguable the most “comeback card” in Faeira. It costs 9 mana, sets your mana to 0, takes half your life, destroys EVERYTHING, then ends the turn not allowing you to do anything more. It fairly decisively resets everything and the sooner you play it the more it hurts you. It is reserved exclusively for getting a second chance. This is how all “comeback cards” should work. You should never be able to play it to break a stalemate or to hold an advantage. If that is its role it should be your exit strategy when you already failed once and are heading to a loss to get a second shot at it.
Your arguments would also hold more water if say Frog Tosser only showed up in green decks. Then yeah it would basically be a green card. But no it shows up in many decks and has been called an auto include by expert players.