You look at donations to people from corporations or through lobbyists and they don't look that large, $15-20k and they are getting big bills passed.
That $15-20k isn't buying the politician or their vote. The threat to give millions of dollars to the opposing politician that will support them is.
The people who are hurt by not having free taxes certainly can't afford lobbyists, and even if they could... they would have to spend so much more than they pay in tax preparation fees for their whole life to compete with TurboTax money. Who would do that?
If they can afford TurboTax, they can afford the lobbyist.
You might say that you should donate out to the fund out of moral principle of a just cause, but there are a million just causes, a million small rent seeking industries that do this sort of thing. Which of them do I donate to fight against? Why income tax and not one of the others?
TurboTax is going to be willing to spend a LOT of money to fight this... you are going to have to get a LOT of people to choose your moral cause, and donate to the PAC.... and then find even more when TurboTax spends even more on lobbying to counteract your PAC. Who is going to give up first?