The problem the Thargoids have is the size of the bubble.
There are 20,000 systems. If absolutely everyone ignored the Thargoids, then they'd take out a few systems a week at current rates. After an entire decade, they'd have taken out maybe 10% of the bubble. On a week-by-week basis, unless you either really enjoy fighting Thargoids, or they're attacking your home system, you can quite safely ignore them with no major difference to the result of the war.
There are a very small number of systems which are really important - Shinrarta, the engineers, a few others. Any individual one of the rest can be allowed to burn with no real consqeuences.
The problem the players have is also the size of the bubble.
If the Thargoids attacked 100 systems a week, they could take out the entire bubble in four years, and irrevocably damage it within three months if they prioritised key manufacturing systems, to the point where you'd need to ship repair materials from Colonia.
But if the Thargoids attacked 100 systems a week, even with a massive increase in the number of players fighting them, the chances of coordinating well enough to save even a tenth of those systems would be very small. Similarly the repair teams would need to be huge to even consider making an impact.
Thargoid fighting is also an advanced activity. You need to be reasonably good at combat and need a reasonably good engineered ship to be much use at it. So new players are always going to need a space "outside" the war that they can get their ship and skills developed. Most players aren't at that advanced level, a lot of players only play occasionally, etc. The pool of possible Thargoid fighters is fairly small, even for just taking out scouts with AX weapons.
There's "living universe" and there's "I came back to the game after a six month break and every system near me is on fire and full of dangerous aliens, how can I get back to safety?"
There's arguments over whether Thargoid fighting could be made easier / more convenient / required in infested/incursion systems etc. but I'm not sure that matters compared with the overall strategic picture: if Frontier makes the Thargoids attack enough that player action can't hold them to a near-stalemate, then they risk the inevitable result being the complete collapse of the bubble in the medium-term [1], probably quite a bit before they'd normally want to end the franchise. On the other hand, if Frontier doesn't make it possible for the Thargoids to do that, they never look like a serious threat and they remain "optional content" in all senses regardless of what they do in the few systems they occupy.
[1] Or needing to pull a deus ex machina out to have the bubble saved by some NPCs regardless of what the players do, in which case the Thargoids were ultimately still entirely possible to ignore after all.