The idea that the Thargoids War is a self contained, self regulating entity seems unlikely to me if Frontier need to inject updates to move it along.
It can be both. If Frontier end up deciding "no more updates" then the Thargoid War as-is will still carry on indefinitely (and on the current balance reach a stalemate position at roughly the current size) with the Thargoids making moves according to the current strategy. It won't be novel but it will provide a permanent semi-dynamic battlefront for people who like fighting Thargoids.
Equally, there are several updates Frontier might intend to make that couldn't be put in place for the start of U14 - the ability to repel Maelstroms and the Thargoids to add (or reposition) them has been hinted at in Galnet, for example - and it would likely be a more strategically interesting war with those updates.
On a more general point, one thing I've mentioned before is that systems where the only major non-deterministic input is players do tend towards fairly boring equilibrium if possible - people can generally assess the relative strengths and make sure they only start fights they can win, powerful groups tend to agree borders rather than start giant multi-system wars, while small groups get unceremoniously crushed. The major sources of excitement come when people deliberately do strategically unwise but entertaining things (with enough tactical strength for it to not get immediately stopped).
So even with a fully player-dynamic Thargoid+BGS+Powerplay+stuff integrated system for managing the galaxy, the likelihood is that players would collectively manage it into "let's keep something very like the status quo" (a relatively mild loss-aversion and prioritising of maintenance over expansion would do that) - Frontier will always need to be injecting adverse events to regularly shake things up even if the
nature of those events is different to the pre-U13 "CGs and Incursions" play.