The US has withdrawn its support from the Syrian opposition after the first 3 years because it instead created the SDF by repackaging the PKK in the region and putting a few Arab tribes in it. PKK/KCK is the most organized non-state structure in the region and probably in the world. This structure has a geographic depth extending to Sinjar, Iraq, up to Iran border, and the US wants to create a state here that will destabilize the region.
Training and equipment activities in the 2015-2016 (after SDF project largely started) period were limited to a 500-person project. The goal was largely a figure as delaying tactic and to resolve Turkey's concerns. As a matter of fact, during the same period, thousands of trucks of materials from military bases in Iraq started to be shipped to the SDF structure, especially when Obama assigned McGurk to the Syrian table. US still trying to put terrorists which their organic link with PKK evident, against Turkey as an interlocutor. For the first time in the Trump era, the letter of a terrorist organization leader was delivered to another country president, which it attached to the US presidential letter.
Actually, ISIS gave the biggest damage to the Syrian opposition and it was the part of greater strategy. In 2013, while the Syrian opposition-dominated most of the country, suddenly ISIS appeared and redesigned the political balance by fighting the Syrian opposition in these regions. After 3-4 year international campaign, at the end of 2018, ISIS transferred half of the land it took from the Syrian opposition to the US proxy organization and the other half to the proxy structure of Russia.