Sfz is free. Decent Samples is free. However in my 20 years in the business I think that buying Kontakt is one of the best investments I have made. There is a huge number of free libraries for full Kontakt, and another bunch for a very low cost. Both Sfz and Decent are very limited compared to Kontakt. If suddenly you find yourself converting Wav files to an instrument where I find Kontakt to be the best. And it is fairly easy. That said I think Native Instruments are exploiting customers with their upgrade policy, they are however by far not the only ones.