While that sounds true in part, the part about handleing it before the time to handle it has pass is the point to take from this discussion.
It appears you are suggesting that the merchant has a right to dispute the chargeback (which is kind of true) and that if you wait long enough they loose the chance, or time to dispute the chargeback. This is a little confusing as the true process is that you would purchase something, get it or not, inspect it, find that you feel you have a claim and report it to the cc. The cc then goes and files the chargeback, the merchant has the same amount of time to respond with regards to your claim to get your money back. Thats right, even if you file the charge back on the last possible day (119 day in the example above) they still have the same amount of time to send in their documentation and prove their side of things. In addition they are CHARGED a fee usually for this chargeback process. This fee is charged no matter if they win and the charge is kept, or they loose and you get you money back.
Something to keep in mind also, cc are not in the habit of doing chargebacks based upon your dislike for the merchant. Nor do they typically do it if you authorized the purchase AND got the item, but just was not working. That is a warranty issue and would fall back on their warranty policy. Now, amex is a little different there as they have a "satisfaction" section for their credit card customer purchases. So, the best bet would be to call cc and find out just what you can do, so you can deal with the sellers with confidence and knowledge.