I've seen situations like this before.
CHUCK, a peculiar blend of JAMES BOND and THE IT CROWD, had a serious budget cut for its fourth and fifth seasons. The show lost one cast member and the JAMES BOND style action was reduced in favour of IT CROWD sitcom antics and significantly less action. Characters spent episodes running around their underground bunker base or inside the set of the big box electronics store.
BLINDSPOT, a high action procedural featuring Jaimie Alexander (Sif from THOR) fighting crime, had a serious budget cut for its fifth and final season of 11 episodes. The show lost a cast member. The regular hand-to-hand combat scenes with Jaimie Alexander were lost. Eight of the 11 episodes had no location shooting and few guest stars and were filmed in an enclosed studio space (a bunker, a basement) or the FBI office.
However, both CHUCK and BLINDSPOT saved so much money on bottle episodes that the series finales were lavish productions filled with returning guest stars, exorbitant stunts, extensive location filming, extended action sequences, and where the episodes leading up to the finale felt like low budget web series tie-ins, the series finales themselves felt like big budget feature films.
In both cases, CHUCK and BLINDSPOT elected to keep as much of the cast as they could afford and accepted a loss of scale, special effects, action and location filming. It looks like SUPERMAN AND LOIS has made the opposite choice and chosen to maintain production value (special effects) at the expense of its cast. There seems to be some chance of the departing regulars staying on as guest-stars. However, given the costliness of SUPERMAN AND LOIS' special effects, it's likely that the visuals will also be taking a downgrade.
We might see something like that with SUPERMAN AND LOIS: the season premiere two parter featuring the usual Smallville cast only for Clark, Lois, Jonathan, Jordan and Lex to be shunted into the Phantom Zone or the city of Kandor or wherever requires few guest-stars and limited location filming. Then they escape at the end of Episode 8 and the two part series finale features the full cast and all the savings of Episodes 3 - 8 being put into the series finale.