This is essentially the series of events, to my understanding:

• Idealistic, young, secular, middle-class students form a spontaneous anti-Assad "pro-democracy" movement in the upswell of discontent that was the Arab Spring
• They are almost immediately given massive coverage by Western media, and even approached by Westerners giving them advice on how to conduct their protests (ie the elaborate apparatus goading and trianing color revolutionaries that has existed since the late 80s)
• A disparate network of American and British ex-spooks rapidly organizes a elaborate propaganda apparatus designed to propagandize for all anti-Assad groups and activity
• The destabilization of the Assad government triggered by the Arab Spring protests in turn triggers a massive upswell in hyper-sectarian Sunni fundamentalism that dwarfs the original protesters, which organize into murderous sectarian militias to conduct pogroms against non-believers
• The Assad government rapidly loses control of the situation and its authority effectively disintegrates in most parts of the country. The original middle-class secular student protesters realize what is happening and back the government.
• The United States, its propaganda apparatus now continuing to portray these radical fundamentalist militias as "moderate rebels", begins flooding arms to these various sectarian Sunni fundamentalist groups. These guys are all fighting each other as well as the Assad government, not merely for sectarian reasons but because they are various proxies for rival US-allied Sunni states - Turkey, Qatar, Saudi Arabia - jockeying for influence in the presumed successor government
• The most effective anti-Assad fighters by a considerable margin are al-Qaeda affiliates like the al-Nusra Front/Hayat Tahrir al-Sham and Islamic State, the latter of which rapidly gains influence between 2013-2014 in the vacuum left by the Assad government's disintegration of authority due to being experienced in fighting insurgencies and being highly organized before the conflict even started (due to having its origins in the anti-US insurgency in Iraq). Despite the US explicitly telling the proxies they're arming not to give these weapons to groups like al-Nusra, because those people are literally the most effective fighters this inevitably happens anyways.
• Islamic State spills across the border into Iraq in 2014. The completely unreliable Iraqi military basically disintegrates before them. Only desperate action, IS overextension, and extensive American intervention prevent the Iraqi government from collapsing.
• Russia begins intervening extensively to protect their ally and hit back against American imperialism, as they have been fed up with the Americans since at least 2008 and are now acting accordingly to roll back or resist American encirclement
• The US begins backing the Kurds against both Assad and Islamic State, in both northeastern Syria and northern Iraq, against the wishes of their Turkish allies. The Kurds turn out to be more reliable proxies. Their initial plan is to split off northeastern Syria from the Assad government and form a Kurdish state chiefly in order to deny Assad control of oil resources in the region. This is obviously contradictory with the US's firm commitment to the Turks as a member of NATO.
• Pretty much all players in the conflict unite against Islamic State. They are essentially defeated by 2018. The Assad government and Kurdish SDF are the biggest beneficiaries. By now the tide has turned decisively against the rebels and in favor of the Assad government. At some point (2016?) the rebel forces in southern Syria become pinned in and a massively "humanitarian" evacuation campaign led by the US and Israel either gets them out of the country or dumped into a stronghold around the city of Idlib in Northerwestern Syria. Additionally in late 2017 the recovered Iraq government moves to crush the autonomous Kurdish Regional Government by force to put down any hint of separatism and uniting with Syrian Rojava as a Kurdish state. This pretty much scuppers any US plan to partition off Rojava into a Kurdish state, but they keep the Kurds in limbo about this, deliberately telling them to stonewall negotiations from Assad, for like two years.
• Turkey, which has been heavily supporting the Sunni fundamentalist rebel groups throughout the war, intervenes directly by invading the SDF's enclave in Afrin in early 2018. This is due to long-running concerns about the Kurdish communist-nationalist-separatists that have plagued the country for decades, and them potentially becoming emboldened and strengthened through cross-border cooperation.
• In 2019, the Turks invade Rojava, intending to secure a "buffer zone" all along the border where they can expel the Kurds and resettle the region with their Sunni jihadist proxies. Rojava, who again have been discouraged from negotiating with Assad by the US for years, are cut loose and thrown to the (grey) wolves. In desperation, they cut a deal with Assad where they will presumably be able to negotiate regional autonomy after the war is over. The SAA takes positions with the SDF in northeastern Syria and effectively halts the Turkish-jihadist advance
• In very late 2019 the SAA begins a renewed offensive against the Idlib Pocket. They are rolling them back rapidly and it seems an end to the war is in sight. Then a (probably Russian) air strike kills like 60 Turkish special forces "advisers" near Idlib. This causes the Turks to again escalate their intervention and they inflict severe losses on the SAA, halting their advance.

And that's effectively where we are now. An extremely uneasy truce and stasis.

In this sequence of events there is effectively no scenario where Assad being defeated by the US-led coalition would lead to a better outcome for the Syrian or Kurdish peoples. You pretty much have three scenarios:

1. Islamic State wins. This probably triggers a direct and large-scale intervention by the United States, which will mean prolonged war and insurgency for decades.
2. The Assad government collapses entirely, without Islamic State as the primary beneficiary. Syria essentially turns into a Libya-style anarchy except much worse, as the jihadist proxies for various actors in the region begin conducting sectarian pogroms with no resistance and fighting each other for their share of the ashes. Eventually one faction will win out and impose a Sunni fundamentalist regime that is a puppet for either Turkey, Qatar, or Saudi Arabia.
3. The same as Scenario 2 except the SDF carves out an independent Kurdish state in northeastern Syria in the process. Turkey invades and crushes them, then either annexes the region outright or eventually hands it off to a puppet government they help set up in Damascus.

Assad is the lesser evil in pretty much every way.