C-sets have one week to live. There are 160 K-set cars but 220 H-set cars to cascade into suburban service, plus Bankstown services will cease by some time in 2023. So early T-sets are already earmarkable for the scrapheap.
The ceasing of Bankstown services doesn't mean a reduction of DD rolling stock needed, since those slots will be taken by additional T8 and T2 services. Both of which are longer journeys than the City-Bankstown portion of the T3, hence it will be a slight increase in overall DD stock needed.
Also T4 will have 10 extra trains once the South Coast line ceases using the ESR (instead taking the Illawarra Locals and the Dive into ST).
That lines up with the second B order being for 17 x 8-car sets- 7 sets to replace the C's, and 10 sets as the extra for T4 (although it would likely involve some shuffling of stock between sectors, with 10 of the 8-car T's moving to T4 away from T1 / T9).
They could possibly scrap up to 10 x 8 car K sets now, (following the 7 X 8 C's) since T4 won't be able to take an extra 10 trains until a couple of years time (once T3 is no longer occupying the Illawarra locals), but that's only if enough
Oscars are able to cascade onto suburban use by then ....
(which in turn depends on what the new D's will replace first - Oscars or V's, or a bit of both).
Otherwise, we may just see up to 10 x 8 K's parked in reserve for 2 years, then brought back out for Sector 2, when 10 x A sets shuffle over to T1/T8, thus allowing 10 X 8-car T's to move over to T4 for the service increases.