The traditional AWACS with a large rotating disc, is seemingly outdated, however saying they are vulnerable really depends on how it is used. Sitting on the ground is very dangerous, as it seems, for any country nowadays.
As for replacing with space-based assets... it really sounds like when the airborne photo-recon aircraft were declared unneccesary because space-based would do the job ... SR71 goes away and the need for airborne assets is re-discovered (oops) so keep those U-2's flying!
Smaller systems on biz-jets appear to be doing sterling work, especially flying along border regions of Ukraine and the Gulfstream-based systems have worked well.