at tech 13 the cannons are almost useless I'll leave them sieging, and now I'm thinking, the infantry put in the second row do no damage, right?
Infantry does damage from the second row (subject to the flanking range). Artillery is still quite useful at tech 13, but the problem is that it's less effective than infantry and it's 3 times more expensive. For example, in an even battle of 2CW inf vs 1CW of inf at tech 13 (with equal rolls of 4), the result after 6 phases is 874 vs 802. In the battle of 1CW inf + 1CW art vs 1CW inf under the same circumstances it is 875 vs 816. So infantry would actually do more damage. The advantage of artillery is that it deals more damage in the first 3 rounds (fire), so it's better than infantry if you expect to crush the enemy in the first 3 rounds. Basically, it's a way of making a battle that is lopsided into your favor even more lopsided. If you expect the longer battle infantry is better, at least until mil 16 and even then the advantage of artillery is quite small.
However, there are additional considerations - artillery is more versatile, you can use it in sieges and in battles, so if you are limited by FL artillery might be a better option even if it deals less damage in the battle (though artillery might still be more expensive than going over FL, at least until you are way over FL).
Another disadvantage of infantry is that when CW is not full AI doesn't position it ideally - some units will be idle and other "wonder off" along the second row. So you would deal less damage than with the perfectly filled CW. You would get extra damage from flanking though. In this scenario 1 enemy width of infantry plus some mix of extra infantry and artillery will be more efficient.