This speculative bit of handwaving of how bad it could have been if we hadn't invaded doesn't make for a very convincing argument, does it? Because we simple don't know, and speculation in that regard seems futile to me.
But just look at the number of civilian casulties in Afghanistan to this day and tell me again we made the world safer ...
Isn't it shallow to convince ourselves that we made our life here in Europe safer from the Taliban, while there were still over 3500 dead & 6000 wounded civilians in Afghanistan in 2014? Most of them because of attacks from said Taliban. ...
Sure, I did not want to convince you. Speculation is all there ever is because we have no second world and cannot really try and see the consequences of alternatives - for every single decision. But does it mean speculation is futile? No, not at all. In order to make up a decision you have to speculate about the consequences and the consequences of the alternatives. Speculations are even essential to every decision making.
This is the reason why I think that the war in Afghanistan was right and not a mistake as you argue. The Taliban were a terrorist group and they seized the Afghan territory and tortured the local population. Unfortunately for them they also posed a big threat to the Western World by giving refugee to Al-Qaeda which were very successfull in 2001 in killing thousands of US civilians. Without attacking them in their homeland they might have stricken again many more times and much harder than now IS in Paris (just speculation). But even without this freeing the Afghan population from the Taliban is a really good thing. If you want to know how good, just go there and ask the people what they prefer. If they would rather go back to the Taliban rule. (Actually they would actually prefer to live somewhere else (in Europe maybe) but this is another story.) The only mistake we make is going out of Afghanistan much too early - is my opinion. I don't actually want to convince you of anything - I just wanted to present a different opinion and substantiate it with some arguments to show that this can be seen differently.
About Syria and ISIS: Sure, talk is currently very important but also in the end fight will be inevitable. If you ask me I would most like a mission of as many countries as possible to disarm and break resistance everywhere in Syria and Iraq and to provide the basis for free elections, etc. - basically the same we did with Afghanistan - because (speculation again) the alternative of letting IS exist further is just too dreadful (and was already for the past 2-3 years).
Now actually I see the arabian countries there also in the duty. The problem is basically they (Turkey, Saudi Arabia, Iran) play a quite dirty game. They want the US or the West to do the fighting and then in the end they will still complain about them (and they won't like democracies much next to them anyway). This is a rather bad geopolitical situation and which you normally would not want to do anything if it would not for the refugees and the terrorist attacks.
So for tactical reasons actually nothing might happen and Syria and parts of Iraq might remain the most terrible place on earth for the next 20 years (basically inhabitable). It would surely be worse (just speculation) for the people there but that's how the world is.
It's always difficult. If you apply violence you'll kill people and if do nothing people may get killed in even greater numbers but you can only speculate before you do something. The temptation to just stick the head in the sand is always big.
However one should assume that fighting against terrorists (with ground troops) is most often worthwhile because most people on earth aren't terrorists and I would prefer their rule over the rule of terrorists any day.