There are 2 topics here:

1 is co terminal (SJC/SFO, LAX/SNA) which I have no comment on.

The other is changing A-B to A-C-B which should be fine. There are many times when all the nonstops are booked and the only other option is 1 stop and we should be able to SDC to those. Delta allows this. American doesn't. So maybe Alaska is trying to match AA following OW admission. If so this is a devaluation by Alaska and we should fault them for making a customer-unfriendly change.