The US needs Russia

Although these groups aren't actively looking to establish a worldwide caliphate, once they've established authority in Syria, they are likely to pursue more ambitious goals, potentially taking aim at the US and Europe, as such groups thrive off violence, war and aggressive expansion.
Their extreme interpretations of Islam - whether Salafist (Brotherhood) or Wahabbi (Saudi) - also puts them at odds with western values and renders them fundamentally incompatible.
The opposition is also heavily fragmented, and unpredictable, with regular infighting erupting between factions. This raises doubts over their ability to govern Syria, and maintain a degree of stability.                
Hence, it can be concluded that most, if not all of the non-SDF rebel groups in Syria aren't suitable partners for Trump to work with.
The Syrian Democratic Forces do appear to be a potentially viable alternative; their successful relationship with the US throughout the war stands testament to this. However, using them to end the conflict in Syria does come with its own unique issues, and causes for concern.
Firstly, although the group has taken on more Arab fighters and even rebranded itself, they are still dominated by Kurdish members. This raises questions over their suitability to retake and govern the entirety of Syria, as there will inevitably be a conflict of interest, due to their ambitions for autonomy. 
Also, the SDF hasn't directly indicated they would be willing or interested in doing so anyway.
In any case, attempting to capture all territory held by the Syrian Army and its allies will certainly involve further destruction to what's left of Syria's infrastructure, high civilian casualties and further internal and external displacement of Syria's population.
More than 60% of the population lives in areas governed by Assad, which includes many major, densely populated cities, such as the capital, Damascus, which is home to around six million Syrians. 
Given the lack of a suitable alternative on the ground, Trump's attacks on the Syrian military's assets and facilities only diminishes the capabilities of one of Daesh' main opponents, and the only force which can end the conflict.
Hence, such attacks prolong the existence of the caliphate in Syria and working with Russia and pro-government forces is the only way to end the conflict with a satisfactory outcome, not only for Syria, but also for the stability of the wider region, and the west's security.
 
Meanwhile, the Organization for the Prohibition of Chemical Weapons (OPCW) fact finding mission has failed to meet even the basic procedures of an investigation, thus rendering the Idlib chemical incident report inconclusive and full of gaps, the Russian representative to the OPCW said.
The OPCW report on April’s chemical incident in Syria’s Idlib province was presented despite being inconclusive and lacking evidence. It stated the mission had established the fact of sarin gas use.
Russia’s Permanent Representative to The Hague based OPCW organization, Alexander Shulgin said that this flawed document was used by the US and its allies to once again pin blame for the incident on the Syrian government during an extraordinary session of OPCW’s executive council.
“It was the US-led western group of countries that came forward with the initiative of this meeting. As it turned out their purpose was to make some noises about the culpability of the Syrian government and president Assad personally in the tragedy of the use of chemical weapons in the Khan Shaykhun region on April 4,” he said.
While the fact-finding mission did not have a “mandate to attribute the responsibility, the US and their allies did not hesitate to interpret the results of the fact-finding report as a one hundred percent proof of the guilt of the Syrian government and President Assad in this tragedy,” Shulgin noted.
The most glaring problem with the report, according to Shulgin, is the fact that the mission never actually visited the site of the alleged attack and collected all the evidence from militant groups controlling the area, thus being unable to determine its authenticity.
“The main problem was that the fact-finding mission has not visited either Khan Shaykhun, the alleged place of the usage of the chemical weapons, or Shayrat Air Base, from where, according to our American partners, the Syrian planes had taken off to carry out this chemical attack,” Shulgin underlined.
“We insisted upon this specific provision and with Iranian partners we came forward with a draft decision in order to send a team of experts not only to Khan Shaykhun but also to Shayrat Air Base. The US and its allies did not allow us to have this decision,” Shulgin said.
“And now we see that it was the wrong decision, because on June 26, [our] American partners… made statements according to intelligence data, that there allegedly are some kind of preparations on the Syrian side to launch a new attack and this is absolutely not logical.”
Russian Foreign Minister Sergey Lavrov also criticized the OPCW last week for failing to visit the site of the attack, saying, that it is “acting unintelligibly.” Lavrov also drew attention to the fact that the FFM report provides no information on where the sarin gas that was allegedly used in the attack came from.
Three days later, US President Donald Trump authorized the firing of 59 Tomahawk cruise missiles at the Syrian military airfield in Shayrat from where US intelligence said the chemical attack was launched, ignoring intelligence data contradicting the allegations.
Meanwhile, the Russian Military Police may be deployed in buffer zones in Syria, Russia’s chief negotiator at the Astana talks and the president’s envoy for Syria, Alexander Lavrentyev said when the fifth round of talks started in the Kazakh capital. 
According to him, the issue, including the status of de-escalation control forces, has not yet been finally agreed. The sides will begin setting up security zones within two-three weeks after signing relevant documents as well as establish a joint coordination center to monitor security in the zones. However, the Russian official pointed out that there are still difficulties in “establishing the lines of contact.”
Elsewhere, reconciliation negotiations between the government and militants have started in Jayroud area in Rif Dimashq province. The goal of negotiations is to disarm local armed factions and to allow people that want to settle their status to do this. Those who want to leave the area will be able to do this towards Idlib. However, no agreement has been reached.