TK is a good program (or PK) - but part of the Unintended Consequence is that 'universal access' will come at a cost of wealthy districts sucking away qualified and experienced TK teachers from the 'most needy student' districts. This happened before - CA universal K-3 class-size reduction (the 22 Billion dollar experiment in Why it Doesn't Work in reducing CA Academic Achievement GAP).
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And - in districts like MP and MVWSD, it still does not prevent accumulating Academic Achievement GAPs as the neediest students don't get the Supplementary Instruction hours that they specifically need, K-5, year after year.

The story doesn't make it clear in the text that districts were already required to offer TK to those with birthdays from Sept 2 through December 2. What 's new is 2 additional months. so that TK covers next year for the first time 1/2 of those 4 year olds who might logically be included, but 1/3 were already covered. Districts which offered no program were violating the law on a technicality.

But what makes this affordable for the state is a vast reduction in the 5-12 year old population since 2013 when TK was created for 3 months worth of birthdays. There are way fewer kids in grades K-8 than there once were, so adding a TK level to a school still leaves the size reduced.

This change is long overdue. Demographic changes have made it completely affordable.

What surprises most parents is that TK does not cover the same amount of the day as their preschool or Pre-K. Most preschools and PK schools cover 9-10 hours. (at least pre-pandemic) TK is much less. And you have to win the lottery to get before and after school care onsite. I work at a preschool, and some parents end up coming back for another year because putting together before and after school care, especially for a half-day TK, costs almost as much as private PK. KG parents face the same struggle.