The question of how the new SA20 December 26 start date will affect the Proteas Test schedule at the end of 2025, has a simple answer - it wont because there isn't one.
The fact is that South Africa uncharacteristically do not have Test matches planned for the traditional period at the end of 2025, and therefore there is no clash with the SA20 this time. Season four of the SA20 will run from December 26, 2025, to January 26, 2026.
In February 2024, however, there was a clash and South Africa were forced to make their marquee players available for the SA20. It meant a critically understrength and inexperienced Test team was sent to New Zealand for a two-match series. The Proteas duly lost 2-0 and their squad selection drew much international criticism.
The SA20 though, is the main profit generator for Cricket SA CSA and it has no choice but to place the tournament as the most important on its calendar.
No competing with the big threeCSA cannot compete financially with Cricket Australia CA, England and Wales Cricket Board ECB and the Board of Control for Cricket in India BCCI, and it has little to no choice about prioritising the SA20 over Test cricket.