Some more conventional long distance hikers have used their gear on multiple trips and so can probably give a good idea about gear durability, but after two such myself my sense is that for the most part durability is less of an issue than you might think. Stuff will happen to gear, but more typically not durability type of "failures", something else has happened along the way to tear up or otherwise damage/compromise gear.

For example, my tarptent contrail was carried on most of two thru-hikes, and is still doing fine, and it's not something selected for durability.

My Gossamer Gear Mariposa Plus packs did indeed seem to me to be good for no more than about one standard thru-hike each, i.e., 2000 - 3000 miles, whereas my hiking partner for much of the AT this year replaced his ULA Catalyst (with a ULA Circuit) after he had 5000 miles on the Catalyst, and it was still entirely functional when he replaced it. Not primarily designed as a super "durable" pack, I think that the ULAs are nevertheless noticeably more durable than the more extreme ultralight options.

Recognize also that you might be swapping out some of your gear along the way, so you won't necessarily be carrying every item for all of any given many-thousand mile trip.