2020 is his 5th year option.....The 5th year option for QB's picked in the Top 10 of 2015 (Winston/Mariota) is $20,922,000.
So Goff's 2020 cap hit will be more than $21 mil for sure.
They don't have to re-sign Goff right now but it's coming sooner than you think....but of course it will only be warranted if he continues to improve and gets to the level deserving of a top contract. Nobody is saying otherwise.
The point is, they need to start planning for it because it will leave less money for the rest of the roster....and they are always looking out ahead multiple years in their salary cap planning.
One other factor is: every year the money gets bigger....so if they are convinced Goff is their long term franchise QB....the sooner they sign him, the cheaper it will be. Salaries show no sign of retreating...... Also, they don't want to get in a Cousins situation, getting stuck in franchise tag hell. That would be a disaster.