Turkish people is used to western standards of modern life. Once the sanctions really hurt the population, he is done. He can't rule alone, and those around him will see an occasion to take his place. US will take care of the successor.
Syrian are poor and divided. It is easier to control such population for assad since his clan is big and has the coast while the rest never went to school. Turkey is full of turkish that have access to schools, internet... They won't starve while a erdogan live like a king. Now he has the support of the people because of anti US feelings but once they see he is a problem they will remove him.
Turkey/erdogan is similar to russia/Putin. They are strong leaders that wants power but are making things better for the country. If things goes bad they are done. They will be replaced quickly. Erdogan is done because of his own interests.
Anyway if Turkey is dead, greece could take everything on the north of bosphorus and make easier for Russian to access the mediteranean.
Iran has had every reason and opportunity to attack the Americans in Iraq... why haven't they done it yet? The IRIAF and their rusted F-14/F-5s are no match for the USAF, they know it despite what propaganda they play.
Why attack US ? Just hit the pipelines and oil fields in kurdistan if Syria and russia don't do it first.
Kurds only hope is to reach an agreement with russia. US won't support them for ever. They never supported anyone for ever. They fought 10 years in vietnam, one day they just decided to leave and let their "allies" loose badly. 20 years later they are sending warships for official visits to see vietnam's communists leaders ... it's like the one who said "being an enemy of US is dangerous but being an ally is fatal".
Kurdistan doesn't bring to them anything extraordinary. They have lot bases around there. They control most of golf countries for oil. And the election of Trump showed that a stupid guy can become president in US and do unexpected things like bring all soldiers back home. BTW he always wants to get them away from syria. Now that ISIS doesn't exist anymore, it is only a question of time to leave syria.