I know you are supposed to replace OEM shocks at about 50k, but I assumed these after market shocks would last a great deal longer than that.
I've never heard a figure for mileage that shocks/struts should be replaced at.
50k miles sounds like something from 70's and earlier cars that were equipped with $6 shocks, that even under the most mild conditions would deteriorate to the point they were doing very little dampening and the vehicles handling was arguably unsafe.
OEM shocks/struts on most vehicles are better quality than the typical direct replacement aftermarket. My "guess" would be Blisten would perform better and maybe last longer, Gabriel might perform as well, but NOT last as long as OEM.
The ride and handling deteriorating is the guide for replacing struts/shocks today. The conditions you drive will dictate how long they'll last. Plenty of "modern" shocks/struts that are driven under "mild-typical" conditions, are still safe and tires wear fine with more than 100k miles on them, but likely you'd benefit from a smoother ride and better handling if you change them by that point.
It's NOT unheard of that an uneven section of road, that a person drives over regularly, or one particular speed bump, wreaking havoc on the shocks/struts and wearing them out much earlier than normal, for an otherwise mildly driven vehicle.
Sorry, but we really do NOT have enough information to tell your shocks/struts are worn out or NOT. Like Jeep5253's link says, there could be several causes of tires cupping. Maybe get a 2nd opinion. You could have someone follow you on the highway, watch the wheels at highway speed, see if you find one shaking up and down unusually, that can cause cupping, and that would be balance and/or shocks.