once you open source you cant close it, just an extension. But i can see a performance module that is closed for 1 bit archival (faxes, company records, large blueprints, civil engineering archives, those are all gignantic multipage tiff, or deep zoom files that need resampling or tiling on demand as you zoom/pan)
also deployable or other viewers. thats a huge SAAS niche. pdfs archival are basically tiffs. or OCR'd tiffs with embbedded full or partial fonts
and anything that is not .net is impossible wth c to mix closed and open source ( the vtable) with mixed version , C is hard but possible, and a huge pain to deploy on various platfroms.
but i got screwed by devcomponents , it still works but windows only, but the company went under and didnt open the source. code so github would need a feature that opens private vaults that produced dependencies with license keys, if they are unmaintainted after a time.
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