Why is Israel so keen on keeping the ypg alive?
Thats right as a buffer or a proxy against Turkiye.
Lets talk about what the PKK exactly is
The PKK in general is a radical-Shia extremist group that loves communism in general! Such radicals are normally situated around the Turkish-Syria-Iraq borders. Moreover, such filth survive by kissing the asses of US, Russia, and Assad; in other words, there is some sort of balance in which if one party pissed the PKK off, they will just wag their tail to the opposite party! The US in general uses the PKK as one of its excuses to remain in Syria
IF such group lost its headquarters to Turkey along the Turkish border, then where do you think the PKK will relocate to? There are only 2 options
1- Assad/Russia/Iran controlled areas
2- US controlled areas
The US wouldnt allow the PKK to enroach their filth close to whatever the US has its hands on in Syria; thus, they will relocate to Assad territories. The moment they do the PKK will be at the mercy of Assad, Putin, and Iran which means an even greater headache for Israel since Iran will use them 100% as all Shia radicals that Iran employs against Israel
That is why, for Israel, it is better if the PKK remains near the Turkish border as to avoid extra trouble for themselves