history consists of "wars to end all wars", [but] not a single of them actually worked at stopping future wars. On the other hand just giving in, like many did to Romans, or Mongols [emphasis mine], not too infrequently led to centuries of peace.

Although the Mongols did take as captives individuals with certain artisanal skills (saddlemaking might be one such IIRC) such individuals only ever made up a tiny fraction of the population of a besieged city. The rest were usually killed (and the city burned) when the city stopped fighting at least when the Mongols were operating in Europe (I don't know about Mongolian operations in Asia) because the Mongols had learned from experience that most European city-dwellers could not adapt to nomadic life. (European nomads might have fared better, but there might not have been any nomads in Europe aside from the invading Mongols).

So, giving in to the Mongols was often a very bad idea.

ADDED. I hereby retract part of what I wrote above, namely, "because the Mongols had learned from experience that most European city-dwellers could not adapt to nomadic life". I no longer have an opinion on the considerations that led the Mongols sometimes to kill the inhabitants of a captured city. Moreover, I retract my final sentence, "So, giving in to the Mongols was often a very bad idea."

I'll try to be more careful in the future :)