I think Fdev quiet rightly are trying to achieve a balance of a credible Thargoid war which folks can get involved in, whilst not destroying the bubble too much for those still just wanting to trade and do the normal missions.
That's fair, but the bubble has ~20,000 inhabited systems and encompasses a substantial number more uninhabited ones. It could lose 90% of those with no noticeable effect on trade and missions (if the remaining 10% were chosen reasonably carefully). It could certainly lose well over the <1% the Thargoids had at their peak without any problems at all.
At their original rate of advance, it would have taken the Thargoids somewhere over 20 years to take out the entire bubble, and at least 2 years before they got as far as threatening - and it would just have been threatening, there's no way the players would have sat back and let them have it - any system of more than local importance (engineers, rares, etc.).
Following the first major rebalance (which still left invasions and so on present), that was adjusted to well over a century to take out the bubble, and at least a decade to get as far as threatening the first important system. In practice, with the system strength reductions further out, they probably would have taken considerably longer than that and might have stalled out entirely at about 40 LY after five years or so.
Obviously, any timescale of "five years or so" or "at least a decade" is likely irrelevant. Five years ago the Thargoids were very slowly skirmishing a few bubble systems (and even then, were "allowed" to attack more interesting ones - Deciat and Kamadhenu were both on the menu at times, though of course heavily and successfully defended by players), Colonia had no engineers, the Hydra hadn't shown up yet, the Witch Head enclave hadn't been established, etc. Ten years ago the only people who'd played Elite Dangerous were a few devs at a minor games company.
There was
never any risk of the Thargoids destroying too much of the bubble to make any significant difference to players uninterested in the content.
It's not at all clear what Frontier think the Thargoid war is for:
- if it's to introduce some safely-contained Thargoid content so that people who like AX can have permanent and reliable access to it with a wider variety of scenarios, then the current situation isn't great for that (because too many of them are tied to Invasions), and the Pleiades would have been a better place for it. The entire automated strategic layer was basically a complete waste of time for that objective. Sure, it makes a minor difference around the edges, but the people who don't care about the strategic details don't need them to be there, and the people who do care need them to be more significant than exactly where the stalemate line is.
- if it's to have an automated system which can replace a lot of the hand-managed content like CGs (and it clearly has replaced them), it needs to be capable of generating more interesting situations than "permanent stalemate at the 20LY line a long way from anything anyone other than a few BGS-oriented groups cares about", and they need to commit to their hype of "this could theoretically make a noticeable difference to the galaxy depending on aggregate player actions" a little more than now.
- if it's to have a more dynamic background and in-game participation to a planned Thargoid war plot, and we're currently at the "humans appear to be pushing back the Thargoids, and then..." stage, fair enough but they need to deal with the details around the way "winning", even on a temporary level, is less interesting than "losing".