No obligation, yes. Nvidia are messing things up for Nouveau and don't upstream their own driver. So Linux users don't have any obligation to use Nvidia either ;) I'd say good riddance, AMD and Intel are doing a good job.

Also, imagine Linux didn't enforce GPL. Would Nvidia be more motivated to upstream their driver? Not in the least. They don't care about GPL. What they want is to have a leverage over the market and charge more for industrial usage, by forcing usage of the blob on their terms (which upstreamed driver would prevent by giving ability to use the hardware to anyone on any terms). That's their whole motivation in this as far as I understand.