The zoom climb was not the be-all and end-all. The Chicoms' MiG-21 could and did zoom up to fly past the U-2s but never managed a shoot-down. (Their SA-2s got 5 U-birds however between 1962-1967). USAF F-102s and - I believe F-104s - were practicing intercepts of CIA U-2s in 1958-59 with poor results. They could get up there but the pop-up point had to be carefully planned in order to get the fighter in the correct parameters for a missile or gun attack. If Soviet high altitude intruders were coming over regularly, I'd expect USAF would have honed their tactics. (This assumes US radars were at least as good as Soviet ones at detecting high altitude intruders. The U-2 program in 1955-56ish assumed incorrectly that Soviet radars would only intermittently track the U-2. Someone else would have to comment on the high altitude capability of the Nike ground to air missiles which were ubiquitous in the States at that time.