I fully agree that it would be so much better to find an artist who could can design a few hundred nice icons in no more than 1-2 months for no more than a few thousand $, put all the icons into the public domain, and make a commitment to be available for designing new icons when needed. However, it is just so much unlikely to happen. If you know someone the let us know.
In contrast, paying about $1000 for an existing icon set a maybe few thousand $ for someone to combine/customize them to create icons Slicer-specific features sounds something that can be done. Risk and cost is quite small and the whole project could be completed in a couple of weeks.
Replacing the default Slicer branding (custom style sheet, icons, etc.) is usually among the very first tasks in commercial product development projects (everybody wants his application to look unique). It should be already possible to override the default icon set without changing Slicer’s source (by setting Slicer_LOGOS_RESOURCE variable to your custom resource file).
Maintaining two icon sets within Slicer would be too much work (we already struggle with only one), but licensing fee for an icon set for a single developer is usually less than a few hundred $ but may cost much less (roundicons vector native line icon set contains 7000 icons and costs $45).