I think that this needs to be said. Again.
Desktop use has NOT declined.
Note: numbers are for US market only and for adults (18+) only.
What has happened over the past decade, since the advent of the now ubiquitous smartphone, is that desktop's relative percentage for internet use has declined...
however,
I say again: it's absolute use has remained pretty much constant.
Put another way, mobile internet usage time has been added to the mix, it has grown the pie to make room, it has not 'stolen' it's space.
Note: granted, individual usage varies widely; we are comparing broad demographic behaviour.
Back in 2008 desktop internet usage was an average 2.2 hours/day.
In 2015 it was at 2.4 hours.
Back in 2008 mobile internet usage was 0.3 hours (18 minutes)/day.
In 2015 it was at 2.8 hours.
Back in 2008 the average internet usage was 2.7 hours/day.
In 2015 it was 5.6 hours.
The pie grew larger as mobile usage became an addition not a subtraction.
Actually, the desktop portion has grown, not just that 0.4 hours (24 minutes) more a day average
BUT
also because the total adult population of the US has increased since 2008: 230 million to 2015: 247 million; an additional 17-million adults.
Or a resounding additional 6,800,000 MORE hours of desktop internet usage per day in 2015 than in 2008.
Yes, mobile has been a mind-boggling success, it has transformed, it has disrupted, it has become both pervasive and ubiquitous.
BUT
It has not relegated desktop internet usage to the scrapheap. Desktop computing has simply matured and plateaued; something that mobile is showing signs of doing soon as well.
The critical point for webdevs is not that mobile has, necessarily, supplanted desktop, rather it is an additional medium of connection. Each niche and site has to decide how much of what sort of device has been, is, and may possibly be used by visitors to do whatever series of interactions. And adjust accordingly. A business decision.
However, it is critical to remember that something new has been added, not necessarily that something has been removed or replaced. Do try not to throw the baby out with the bathwater.