Tax software would make for a terrible open source project because of the intersection of domain expertise: to do it right you need CPAs and tax lawyers. To do that right, you have to pay them (because there's no "open source" equivalent of a trained and certified CPA or lawyer). If you are paying domain experts, why shouldn't you also be paying software developers? Building a business plan for something like tax software is already hard enough, especially with such entrenched players already, but building a business plan for something like tax software that is also open source is extreme hard mode.