With my work I end up going to many stores, it is the same story.
Irrespective of what is happening online, under your logic if more and more people are buying digital or on amazon, surely all platform sections would shrink accordingly in terms of shelf space. The shelf space arrangements I mentioned have been around like that long before online shopping was a thing in Australia. If these haven't changed clearly it just shows that PS4 has more games that are worth putting on shelf or Nintendo don't want to pay for shelf space.
OK lets roll with your online idea, i checked online for one of the major retailers here in Aus.
Ps4 has 90 announced games coming in 2019, with 50 confirmed release dates
Switch has 40 announced games coming in 2019, with 15 confirmed release dates.
Surely you can see why the Switch looks like its in a dry spell to a gamer that has other tastes then just Nintendo games?
Hell I ain't even worried about 2019 too much its the fact we are not seeing any 3rd parties really saying they gonna go full swing into switch development either which is concerning.
Shelf space is shrinking for all platforms especially here in the UK (to the point many retailers of games have shut down even HMV the once flagship retailer of games and music is in financial trouble as a result and shutting down so your whole shelf space angle is waffle as it's nowhere near a concrete argument in measuring library we're not in the 90s.
Duh PS4 has more announced games it's been out for 5 years and NS is not even 2 years, back when PS4 was out for the same time period it didn't even have 40 announced games and such for the period ahead, I have other tastes than Nintendo and not had the same issue as for your claim about developers:
This is from GDC's survey ahead of GDC in March.