It's not "demonstratable" in any form. The manga industry has been transitioning from print to digital.
It did not take a hit. On the contrary, it improved from 2015. See link below;
Link -
Manga Market improved year on year in 2016 from 2015
People LOVE to say digital has made an impact but how much? Is it 50% like the market value? Is it 30% like Shuiesha's chief's statement from two separate years? You know, Shueisha, the ones who have the largest print circulation in Japan? Or is it 10% or 1%?
Lets look at a manga from almost a decade ago and now;
Toriko - Peak sales I can see is 480k a volume. Was widely considered to be extremely popular. Fun fact, some 15 chapters in, Toriko was placed right up in the front just behind the Big 3 for WSJ's double issue Joint Cover (something I don't think has happened with any new series from any returning acclaimed mangaka).
Had a peak print number of 570k (or similar) per volume
Kimetsu No Yaiba - It's latest volume crossed 500k. It's popularity level was nowhere near Toriko. Even right now, after its anime it hasn't exploded with popularity to the levels of MHA or TPN. Yet, KNY has a higher volume peak and it's print number would probably be between 600-650k which is closer to the likes of Reborn.
How does this fair under your assumption that the industry has taken a hit?
Quality work will get popular and sell equivalent to how great it is. Kingdom basically tripled or quadrupled its print sales in the past 5 years because its a fucking stellar series.
People like to say MHA never touched 1 million because of the era. No. Maybe MHA didn't touch 1 million because it only achieved what Reborn or D-Grayman achieved in levels of popularity. Stop assuming that because a series is SUPER popular outside Japan, it experiences the same level of popularity in it. Many assume MHA deserves 1 million because of how popular it is outside Japan which is such a wrong assumption.
Nakano himself stated two years ago that he wants a new pillar that sells more than a million a volume. Why would he state that if it was not achievable in this "declining era"? He has way more information than you at hand and way more experience.
Everyone needs to stop assuming stuff about digital without any information.