You can shrink die size, you can shrink transistor size, you can even make 3D stacked memory but optics and physics play a significant role in photography. Shallow depth of field cannot even be achieved with such small sensors comparable to larger sensors and yes it has everything to do with one's artistic style when used correctly. I'm not saying it's not possible, it's just improbable to compete with a full sized 35mm sensor - hell, even a 1" sensor. Unless you use software tricks to get the effect you want, however.
Not to mention that while you can improve the sensor, in any given sensor generation, larger sensors can afford larger pixels, which means better low light performance. Certain kinds of photography cannot even be done on a small sensor due to the low light performance (astrophotography). Other kinds cannot be done because the camera on a phone simply lacks the ability to be controlled in certain ways.
But to answer the questions directly:
Physics says smartphones can never surpass tablets, tablets can't surpass laptops and laptops can't surpass desktops. Which will never be as powerful as mainframes. Who do you know still using a computer as big as a whole room?
Many people still do. Just not directly. Who do you know uses Dropbox (or similar)? Google Search (or similar)? Then your use of computers still depends on these big monstrous clusters of CPUs. Even if you don't have one in your basement.
When the sedan appeared, did trucks disappear? No. The right tool for the job is still the right tool for the job.
The progress of technology changes the size of the best devices. We already have 12mpx, 4K video, 240fps slow motion, OIS, Live Photos. Is it so hard to believe that DSLRs will once have no decisive advantages anymore?
Yes, it is hard to believe. Camera phones may democratize photography, but it doesn't make DSLRs obsolete. Far from it. Many of the technologies that smartphone cameras demand in order to not be terrible wind up making DSLRs that much better, or are derived from DSLR advancements. This isn't a case of catchup, they are evolving in parallel and learning from each other.
And I don't really agree with the premise that the progress of technology changes the size of the
best devices. That metric is subjective. The progress of technology provides for more options, more access, and more democratization of technology. So the size of what is
most popular, or
most used changes (by the nature of more people being able to participate), but that may not even change what is best
for a particular use. I use my smartphone camera, but I use my mirrorless DSLR to take photos that are impossible to capture any other way, except maybe on a Film SLR.