The game launched with a very small set of game play mechanics. Since launch, they've removed 1 mechanic (tracking pokemon) and have added nothing .
If they were capable of keeping their launch-day mechanics in place and weren't scrambling to just keep the servers alive (the reason they removed the mechanic they did) then they could have focused their small development team on improving the game instead.
The key mechanics in the old pokemon games was battling friends & AI and trading pokemon. If they added those mechanics into Pokemon go, then they might be able to keep the interest going a bit longer.
Until then, it's collecting things that you can't find. The fun in that wears thin pretty quickly.