Some people keep saying but I have yet to see any personal evidence for this alleged "trend". All of my relatives, all of my friends and all of my colleagues (with not a single exception!) have a PC and a tablet in their household, if they own a tablet. I have never heard of anyone who uses a tablet but does not use a PC or laptop at the same time.
I also highly doubt that there is any statistical evidence for this trend, according to which clearly more people have a tablet and no PC/laptop than there are people with tablet and a PC/laptop. Probably for mobile phones but not for tablets. Tablets are additional throwaway/low lifespan gimmicks rather than replacements of PCs and laptops.
There are some, but mostly those are people who aren't using personal computers to produce in the first place. In the early-mid 90's a common refrain (I remember my parents even saying it at one point) was "Why do we need a computer?" For those people, virtually everything you could respond to answer that question with (intercommunication with others, organizing, entertainment, writing text documents, etc) is served by a combination of smartphone and/or tablet with access to internet-enabled applications. Maybe a tablet + keyboard (or a Chromebook) for extended writing sessions.
The business users, academics, and developers are still there, but they now make up a much smaller fraction of the overall computer market. When you add back in enterprise users where corporate policy aligns well with thin client / network computing paradigms, you get an even smaller fraction that needs local personal computing... basically just those above, plus those who need or want local control for reliability, network reliability, custom performance (eg, gamers) or philosophical reasons.
So a need for local apps will still be present for some folks, but the large surge in the late 90s and 2000s doesn't have a need for a true PC. It's a shame, because it increases the barrier to entry from consumer to developer (Apple products, ironically were great at *reducing* that back in the Hypercard-installed-on-all-Macs days), but it's good because it lowers the barrier to entry for access to computing resources generally.
tl;dr: Desktop apps (and "personal computers that aren't smartphones or tablets") are going to shrink back down to the market they were before the late 90's. Congrats, all of us, on becoming geeks again. =)