When you look at people in the streets, or actually everywhere, they go with the head down in the smartphone. How many likes did I get at Instagram, or FB? When I am out I never use my phone. It's only to if people call me.
Smartphones are fun, but they are limited, purely due to their form factor.
If the GPU and CPU OEMs are having a hard time producing powerhouse chipsets (16 cores @ 7nm) to perform production, compiling, and run-time tasks in the developer world, then smartphones are way out-classed. As far as gaming goes, console games would choke most phones, and then you have the RT (ray-tracing) games and media content and all their glorious eye-candy, sheesh...RIP tile-based deferred rendering.
No matter if we are discussing software or hardware, I always say use the right tool or set of tools to accomplish the task.
And as someone else previously stated in this thread--smartphones, laptops, desktops, and tablets all complement each other and each performs a certain necessary need.
ie. I would not want to perform studio-grade video production and editing on my phone or tablet...gorilla style production sure.
Even when I use a drone or a high-end camera (for work or play) those files would be a PITA to edit on a phone...dropping or cleaning up audio that I or my clients don't want in the video would be a daunting task. I would hate trying to use a spectral analyzer or parametric equalizer to scrub audio files.
Coding/scripting/programming on a phone is ok, but not when it comes to multiple class files and thousands of lines of text I reach for a 13" laptop at the least.
I perform minor updates to servers that I maintain from my phone but I'd rather do the heavy lifting from a desktop or laptop.
There are certain tasks for wireframing and even intermediate graphics creation or editing that I will start from my phone or tablet, but will finish up on my desktop or laptop.
Depending on the situation I will use what I have readily available if it is a crunch situation. I have been known to walk into an Apple Store create work on one of their desktops and SFTP the file(s) via terminal out to my client or a content aggregator/CDN/server.
But what do I know, this is just my 2cents amigos.
I don't know if it will be due smartphones or due to a combination of smart devices and SaaS services but ultimately yes I think Desktops will have the same fate as dinosaurs.
Personally I hope they'll stick around as I'm too used to having a "PC" and don't like using a smartphone nor a tablet, plus I prefer running the software on my drive with the data staying on my drive, SaaS in a sense is like "dumb terminals strike back"