Campaigns are based on time, so it does not matter wheter you finish them or not. When the time is reached - the campaign will swithc.
Some things/upgrades might be unavailable (sorry, haven't played an unmodded SH5 for forewer, so can't exactly say what and where), but overall you still will progress.
I myself still haven't finished the entier game, though have it for the past 2 years. I blameTDW, Trev, sober, soianm, gap and many many other amazing modders: they make em so irresistable to install that I have to restart the campaign every time....
