Actually in Vietnam the US faced air defense that is compare to contemporary airpower as formidable as any ever faced in combat by anyone.
Vietnam did win a war due to this air defense and problems that they had created for US, this is why US are MUCH more careful when they choose what country to invade and this is why they block countries from obtaining decent air defense.
As for Israel, it is pointless to discuss, whole region is hopeless, Saudi Arabia failed to defend from Houthi. Up to the point that they were forced into ceasefire, and this is despite having modern supplied by US air defense network.
As for SEAD. It is not as simple as you think if enemy can use radar installations and AWACS from allies that cover most of territory from the border. If air defense units do no use radars except time that they shoot on target, it is extremely hard to destroy.
Even Serbia managed to keep it air defense units intact and make some damage to overwhelming NATO air force with outdated equipment by using ambush tactic. And it is without allies providing them with radar live feed.