Maybe taxing on the distribution of proprietary software would be more palatable? After all, software that is written can only become proprietary if you distribute it to other people under a non-free license. I personally think the warranty idea is much softer on companies (while still giving some more protection for end-users).
I don't think banks should be taxed for having propreitary systems. I do have a problem with SaaS[1] companies, and companies which make money of selling software which is proprietary -- because they are actively creating a monopoly on the expertise in and ability to support their particular software (known more politely as vendor lock-in). Not to mention that proprietary software developers incredibly often mistreat their users through a variety of schemes.
[1]: https://www.gnu.org/philosophy/who-does-that-server-really-s...