From the article:
"Some 14% of U.S. adults have a portable gaming device such as a PSP or Sega Genesis game player..."
Ladies and gentlemen, the Sega Nomad!
Also, the headline/OP is confusing stagnant with "dying". The same article describes how smart phone usage is reaching saturation with young adults, but doesn't say that it's "dying" with that group, implying that they know the difference and are trying to advance a narrative.
IOW the stats are interesting, the headlines are misleading. It would be better to say "Survey reports no growth in dedicated consoles over the last 5 years"