If the vendors got metrics that showed that desktop linux user share was enough and viable enough, they would. The issue is, Linux users are a small share of the desktop market. But, if that number grew, it changes. Hardware vendors would port their drivers over once the potential revenue of customers on a different platform outweighs the cost in doing so. Otherwise, I don't think Brother or Nvidia really give a damn if an end user is running Windows or Linux, they just care that enough of those user exists in their respective ecosystem to justify the engineering costs.

This is true, but still - people want to print documents. They don't want to run Linux (or Windows). If they can't print documents, that's a problem.

The solution was Google Cloud Print, IMO. That was a great idea. Who cares what you run, as long as it can connect to the internet you can print. I don't know how in-depth it got with the various crazy menu systems

That brings up an excellent point. Services like these start to eventually render driver support useless. I haven't heard of Google cloud print, but I just looked it up. Even though it looks to be EOLed, there are alternatives still going, just not from google. My HP printer has an app. I can push documents to the app that goes up to HP then comes back down on to my printer, if I have my printer configured with that. I think my last printer, which was a bother had something similar where I could set it up on my home network in such a way that I can just email a document to a special bother email address which then prints it out on the brother printer.