As so many are concentrating once again on individual trees, here's what the forest looks like.
The immediate consequences of the US withdrawal in the north and subsequent Turkish attacks are the possibility of ISIS and Al Qaeda associated prisoners being freed and those movements reorganizing in Syria in the power vacuum created in the space among the warring parties, harm to America's Kurdish allies in the north with the possibility of large scale casualties among both soldiers and civilians, and harm to US credibility as an ally more broadly.
The latter impact, that as an ally, is somewhat mitigated by the fact that it has been similarly problematic for the US to work so closely with the Kurds as an enemy of our NATO ally, the Turks, regardless of the problematic nature of Turkey's current government. The US has substantial military and economic connections with Turkey and the US actively standing against Turkey in favor of the Kurds obviously is and has been an issue for the Turks.
Another thing that is often lost on those looking at the situation in Syria is that it is a multi-lateral conflict, not a simple one. This isn't simply a conflict between the US, Turkey, and the Syrian Kurds. Just to list some of the other major players involved in some way, we have the Russians, the Syrian government, Iran and the IRGC, Hizballah, Da'esh, Al Qaeda affiliated groups, Muslim Brotherhood affiliates, Iraqi Kurds, who will aid their Syrian brethren, the Iraqi government which faces instability already, Israel, Jordan, which could face a worsening refugee situation, and the Saudi, Egyptian, and UAE anti-Muslim Brotherhood anti-Iran alliance. Each of these will interact with other parties in the conflict and if the situation deteriorates further will become more involved.
Those American troops in Syria's south at Al-Tanf were/are there to prevent Iran from controlling a land route that would enable the rapid build up of military strength across a wide swath of territory. That land route could destabilize Israel's northern border, allowing Iran to move large amounts of missiles and troops into threatening positions. Stability in Al Tanf also limits the flow of jihadi fighters toward the Israeli and Jordanian borders.
Should ISIS and Al Qaeda fighters escape from Syria, many will bring veteran military experience, IED expertise, and a commitment to become a martyr with them wherever they go. It would not be surprising to see increased terrorism in Syria, Iraq, the Sinai, and Europe, but also, with fighters returning home from a number of countries, new or worsened problems in other nations as well.
In relation to Israel and Turkey specifically, the two nations have seemingly had an agreement, that the Turks would not aid Hamas and harass Israel as long as Israel did not actively aid the Syrian Kurds against Turkey. The possibility of dramatically increased threats against Israel coming from a destabilized northern border and substantially increased threats from those enemies of Israel involved in the conflict will compel Israel to become more involved. That Israelis are overwhelmingly in support of the Kurds and are greatly angered by what they see happening in Syria, may lead the government to act in support of the Kurds. This will almost certainly not be with open action against the Turks, but may include action against their proxies and will likely include replacing the US in relation to training and potentially helping to arm Kurdish forces.
Turkey may then renew hostile actions in relation to aiding Hamas, but the Israeli public is further along in support for taking more substantial action against Hamas. It is reasonable to conclude that if Hamas launches an offensive, Israel will respond more forcefully than it has in recent times. This would be especially true if the threat of a second front in the north becomes more of a concern. Without a US presence, the likelihood of flareups in Syria leading to the possibility of an Israeli-Hizballah or Israeli-Iran conflict increases substantially.
The SDF leadership has threatened to negotiate an agreement with Russia or the Assad government. Just take a moment to consider that not long ago, the United States was in a place to mediate between Turkey and the Kurds with both wanting to please the United States. Should the Kurds end up seeking protection from Russia, we will be looking at a situation in which the Kurds protected by Russia instead of the United States would be threatened by a Turkey with Russian air defense systems and seemingly now in the Russian sphere, instead of using NATO air defense systems and in the American sphere. Russia would have fully replaced the United States' role in Syria and in regard to Turkey.
As Turkey will almost certainly see the opportunity to create a buffer zone against the Syrian Kurds as more valuable in the long term than the damage that would be done by even extreme US sanctions in the short term, the US will have to impose the threatened sanctions and relations between the US and Turkey will severely deteriorate even after the US conceded to its "ally" the ability to act in Syria. Thus, the act of giving in to Turkey here may result in the certainty of a fully deteriorated relationship that was only threatened by US shielding against a Turkish invasion in Syria.
If all of this comes to pass, the likelihood of the United States needing to send troops overseas to defend US interests increases substantially and with the resulting conflicts requiring more US resources, both in terms of human assets and equipment, than the Syrian special operations have required. This may be true in Iraq alone.
In other words, the removal of a limited number of soldiers in northern Iraq could lead to a deterioration of the region that would result in the United States expending far more resources and in America and all of its allies being worse off. This all should have been determinant in foreign policy decision making. Because it was not, the dry tinder has already been lit.