On Day 2, with external ordnance and drops, the F-105D goes way farther, waay faster, and drops waaay more bombs than a clean F-35A, no? Add externals to the F-35A, and now the F-105D goes waay farther, waaay faster, and drops way more bombs than a F-35A. Isn't that right?
Let's just talk Day 2 for awhile.
Vietnam was fought for many years, so "Day 2" and thousands of days thereafter. A LOT of Thuds are still in the jungles of Vietnam. They had essentially zero air-to-air capability and had absolutely zero precision ordinance delivery capability. If you want to drop a lot of iron bombs with what today would be considered miserable accuracy with questionable survivability, then the F-105 would be a very fine platform to do that.
Edit: The A-7 was a fine attack platform also and replaced the F-105 even though it was much slower. It was operated by both USAF and USN, plus a few non-US air arms. And the A-3 in USN service, (B-66 in USAF service) was a pretty good bomber that could deliver lots more bombs farther than either the F-35 or F-105. The A-6 in USN service and F-111 in USAF service were fine attack platforms as well in their day and the F-15E/I/S/K/SG/SA/SE continues to be an exceptional attack platform today with a bigger payload and greater range than F-35. F-16 and Rafale are also a current production really good attack platforms. And the Typhoon is getting there. But clearly USAF, USN, RAF, RAAF, CAF, IDF and several other air arms have requirements that those platforms cannot meet effectively and that's the very reason the F-35 exists. It does things the others cannot do and clearly there are a number of air arms willing to pay a premium price and wait much longer than planned to get the capabilities the F-35 brings to the table.