If the Thargoids wanted us to leave the Pleiades, they could have just asked us to leave. They didn't, therefore there's no reason to believe that the Thargoids will be satisfied with just that portion of the galaxy.
Ever think they are having as much difficulty understanding us as we do them.... the circumstances are not unprecedented...
Think of the "Darmok" episode of Star Trek TNG, where the alien race language is metaphorical based communication and neither they nor Star Fleet could understand each other until one side made a break through in understanding the other.
Or alternatively, there was that movie Arrivals with those squid like aliens and that strange pictographic language (with a temporal element IIRC) that was incredibly hard for most humans to understand.
There are even more examples than that but I think the above two examples illustrate my point adequately enough.
When attempts to communicate fail, sometimes violence is the only way to indicate desire/intent.
We don't know much about the Thargoids and even less about their intentions but what we do know is:-
- Thargoids were driven away or killed as a result of a human made WMD
- Thargoids have been absent from (at least human known sections of) the Milkyway for long enough that they became considered the boogie-men of myth and legend
- Barnacles started appearing in the past 2 years
- Humans started mining these barnacles for valuable materials
- Humans started expanding into the barnacle populated regions
- Thargoids started appearing in the region of space where barnacles have been seen
- Humans became even more interested in the barnacle region
- There have been some violent encounters between major faction fleets and the Thargoids
- Humans started developing weapons and equipment to combat the Thargoid technology
- At least some humans engage in combat with the Thargoids and winning in at least some encounters
- Thargoids start attacking newly established human stations in the barnacle region
Whether the Thargoids would have attacked us even without the Barnacle mining and expansion is unknown and pretty moot now. If we back off from the region and let the dust settle rather than press matters further then the least it will do is give us a chance to consolidate our position outside of the region they have an obvious interest in. If they then start encroaching on our space then we might be collectively in a better position to respond and defend ourselves.
We have no inherent right to the Pleiades region nor the resources there, the Thargoids on the other hand may have a stronger claim.