Crops are limited by temperature (both high and low), soil moisture, day length, duration of the growing season, etc. For example, the upper limit for wheat is about 35 degrees C. Climate change will impact rainfall patterns as well as temperature, so some areas that are currently suitable for a particular crop will become unsuitable as new areas become suitable. It's not a given that the tradeoff will be balanced. Also even if northern areas warm their day length will not change. No matter how warm it gets, above the arctic circle you'll still have months where the sun doesn't get above the horizon and lots of very short days in the spring/fall resulting in a too-short growing season.
I don't really understand the perspective that says that changing our energy economy is too much bother, and we will probably be OK for my lifetime, so we'll just carry on and hope people 100 years from now can work it out. If not, it's not my problem.