On further thought regarding the possibly always fairly thinly populated nature of Arnor, at least concerning non-Dunedain: Appendix A says about Earnur's northern expedition: "So great in draught and so many were his ships that they could scarcely find harbourage, though both the Harlond and the Forlond also were filled; and from them descended an army of power, with munition and provision for a war of great kings. Or so it seemed to the people of the North, though this was but a small sending-force of the whole might of Gondor."
While Gondor at this point was much larger and more powerful than it was at the time of the War of the Ring, it was already well past its peak (post Kin-strife, post Great Plague). If "a small sending-force" of post Plague Gondor was really huge by their standards, that might suggest that the Arthedain vs Angmar war was fought by pretty small armies on both sides - maybe more like Migration Period warbands than full-on armies. The Dunedain were well diminished by this point, but the non-Dunedain humans wouldn't have been touched by that. But if they were never numerous to start with...
(The fact that the very good farmland of the Shire was abandoned or mostly abandoned by 1601 TA, so that it could be granted to the Hobbits, also leans in that direction. Possibly since Angmar was northeast of Arthedain, the Kings of Arthedain had to move their apparently limited population eastward to maintain the lines against Angmar - and possibly Rhudaur - leaving western areas like the Shire fallow?)
While Gondor at this point was much larger and more powerful than it was at the time of the War of the Ring, it was already well past its peak (post Kin-strife, post Great Plague). If "a small sending-force" of post Plague Gondor was really huge by their standards, that might suggest that the Arthedain vs Angmar war was fought by pretty small armies on both sides - maybe more like Migration Period warbands than full-on armies. The Dunedain were well diminished by this point, but the non-Dunedain humans wouldn't have been touched by that. But if they were never numerous to start with...
(The fact that the very good farmland of the Shire was abandoned or mostly abandoned by 1601 TA, so that it could be granted to the Hobbits, also leans in that direction. Possibly since Angmar was northeast of Arthedain, the Kings of Arthedain had to move their apparently limited population eastward to maintain the lines against Angmar - and possibly Rhudaur - leaving western areas like the Shire fallow?)
