PT: 2009
HGSS: 2010
BW: 2011
BW2: 2012
XY: 2013
ORAS: 2014
SM: 2016
USUM: 2017
LGPE: 2018
SS: 2019
I seldom use this word, but four years without a mainline Pokemon game legitimately feels impossible to me.
Can I remind people that this is the following scenario
When the game that is the start of a generation is completed, work immediately begins on the next generation. Not the next game, the next generation.
While development of the next generation is in play, smaller teams led by the younger staff (at points when they wouldn't be needed for the big new generation games) work on smaller projects such as Enhanced Versions, remakes and now DLC instead of Enhanced Versions based on the framework set by the game starting that generation so less work is required.
As such, Sword & Shield started development in 2016. Sun & Moon in 2013 and so forth.
4 years between SWSH and Gen 9 is possible, we've even had gaps that long before. Ruby & Sapphire to Diamond & Pearl was 4 years and Diamond & Pearl to Black & White was 4 years.
Also your list is slightly askewed. We should go by original releases
PT: 2008
HGSS: 2009
BW: 2010
BW2: 2012
XY: 2013
ORAS: 2014
SM: 2016
USUM: 2017
LGPE: 2018
SS: 2019