An interesting note about those new industries you mentioned: they're all contractors. When people talk about working for Uber or door dash they typically aren't saying 'this is what I want to do for the rest of my life', it's more of a holdover until something better comes along. As these individual companies begin the process of automation it may be that contract work is what most of end up doing. Once most of us are contractors it will become a supply and demand situation where we all seek to underbid one another in order to feed ourselves and our families. We would still be working, but it would be like fighting over scraps.

If 90% of the workforce was suddenly laid off and left to starve, what do you actually think would happen? That we'd all just sit at home and quietly die? Ask the french royalty what happens when it's population realizes that it's main hope to not starve to death is to dismantle the existing system and start over.

You're right, of course, but I doubt that it would happen suddenly. The process of automating 90% of the work force would likely take decades and be a long, slow process with a lot of half measures a long the way to appease the masses, much like we experience today. I imagine full-time work will be redefined to fewer hours and eventually we will need something like UBI to supplement us and drive the economy. Tax burdens will likely shift to corporations in order to keep the government running as human labor will slowly phase out.

And there's only profit to be had in any case if there are people with money to buy things.

I think that this is the crux of the argument. As automation becomes cheaper than human labor, human labor becomes intrinsically less valuable. This means that any paid work will simply pay less, which gives the lower classes even less purchasing power. Wealth concentration will continue to worsen and the middle class will evaporate. If capitalism continues, it is at this point industry and economy will revolve primarily around the needs of the rich. The people will still be a consideration, of course, but more of a liability than an exploitable resource. A world war ending in nuclear holocaust would likely solve that particular problem, but I'm hopeful that capitalism will be abandoned before it comes to that.