Discord will always have a market as long as competing marketplaces / ecosystems exist on PC. Discord's platform agnosticism makes it a safer bet than the three companies you mentioned, all of which are incentivized to build closed content ecosystems that antagonize each other in order to move more product. There is simply no other solution that allows me to see what my friends are playing, jump into their channel, and see the gameplay broadcast live from multiple perspectives in a private environment.
The Netflix analogy is not particularly apt here either, as where Netflix produces the content, here the content is user generated. So its about technological executions, platform agnosticism, and features sets. Valve's Steam app on iOS didnt even support iPhone X screen resolutions until a year and a half ago so I wouldnt hold my breath for them to compete well in this space at all. Maybe Epic's acquisition of houseparty could cause some competition for them? But I doubt it.