The vast majority of air defence systems the US has gone against are SA-2 which is late 1950s tech, SA-3, which is early 1960s tech and a few SA-5s and SA-6's, which are late 60s tech.
Over the past decade and a half, things have started to change. Russia has started exporting large numbers of modern air defence systems, and modern digital upgrades to older systems. Systems that were designed from the start to be highly resistant to jamming and decoys and to be highly mobile.
Air defences have suddenly got a lot more capable in many nations around the world, while western nations are by and large still using the same defence suppression tools they had during the first gulf war.
If the US needs to attack a nation like Iran or Syria, it will not be a rerun of 1991. Instead of the US having a 20 year technological advantage over enemy air defence, enemy air defences will have a 20 year technological advantage over the US air force.