While Thieu's meddling in the defense strategy during the Spring Offensive was certainly a direct factor in the collapse of the Central Highlands, it was only one of many. In a way, you could say that indeed the conditions were right for the Republic of Vietnam military forces to fold in the face of overwhelming conventional force.

When you dig into it, the position the South Vietnamese found themselves in was pretty much no-win by any realistic appraisal, however. By that point, in the Central Highlands, ARVN artillery was being rationed to six shells for 105mm pieces, four for 175mm pieces, per gun, per day. Hand grenades were strictly rationed and infantry had their small arms ammunition load reduced by 50%. In some cases, like in MR I, soldiers were supplied a single hand grenade and 85 rounds per month. Additionally overall, medical supplies were so lacking, bandages and blood bags were being reused. Even a coherent defensive plan and fighting retreat would have collapsed with no ammunition to use.

Additionally, severe cuts to fuel supplies and spare parts had rendered much of the VNAF immobile. Helicopters which had once been a primary part of the ARVN's tactical mobility fell to the wayside and trucks and feet became the main means by which the South Vietnamese moved—the exception being the units who made up the South Vietnamese strategic reserve: the ARVN Airborne Division, Marine Division, and Ranger Groups.

And one oft-ignored aspect of the collapse of the Central Highlands is the context in which the South Vietnamese forces lived and fought: the regular army divisions often recruited men from the provinces in which they were stationed. As a result, families could remain close at hand and in some cases, would follow the division around on deployments. When Thieu ordered the withdrawal, which came as a complete surprise, the idea of desertion, which the ARVN already suffered greatly from (due to economic recession, the South Vietnamese government was forced to successively slash enlisted pay), became quite attractive as men thought about getting their families out of the path of the North Vietnamese offensive. This would result in the frenzied evacuation along Highway 1, also known as the "Highway of Death."

And yes, Thieu's decision to withdraw from the Central Highlands was tactically disastrous for all of the obvious reasons: virtually no one lower than the JGS level was aware that it was even being mulled, and so it came as a complete surprise to field commanders, who were given scant few hours to immediately plan for it. Now strategically speaking, it made a lot of sense: why waste men and resources trying to defend the entire country when most people lived along the coast in a few cities? Well unfortunately, Thieu rightfully knew that it was a politically untenable situation to abandon a geographic majority of the country, which shows just how desperate the situation had become.

For further reading on the military factors in the collapse of the South Vietnamese military forces, I can't recommend George J. Veith's Black April: The Fall of South Vietnam, 1973-1975 enough. It is the comprehensive source on the matter.