if your international trips are in paid business class there is really not a reason to mileage run to 1K, since the biggest distinction between the two levels of status is the GPUs. i guess you also earn a couple of more miles per dollar spent on tickets too as a 1k vs. plat.
at any status level, all revenue fare classes are upgrade eligible. you're correct that for an instant upgrade at time of purchase you must buy YB as a plat/below or YBM as a 1K, and you also have to have availability in the instant upgrade fare bucket (PN as i recall).
another distinguishing factor between gold and plat/1K is that for plat you get 2 RPUs and for 1k you get 4, plus another 2 for every 25K additional miles you fly (i.e., 125K = 6 total RPU).