The Astros have reached full maturity as a franchise. From full rebuild before 2015, to contenders, to youthful champions, to continued excellence as a veteran team, and coming soon, a slow decline. The youthful champions of 2017 are ready to get fully paid, and the Astros can’t afford to keep all of them.
Alex Bregman and Jose Altuve, half the Astros Core Four, are signed to long-term contracts. That leaves George Springer and Carlos Correa left to be re-signed. Springer becomes a free agent after this season. Correa enters free agency after 2021.
It is highly unlikely that the Astros can afford to re-sign both.