Radak: Even if people would live long lives, they could not fathom His greatness in all their days - and all the more so, since man's days are short! What can they do in their short days? "Dor Holech v'Dor Ba" (Koheles 1:4) - before a generation goes, it will tell to the coming generation, the awesome deeds that they saw in their days, and the mighty acts in their lives, in order that the coming generation can praise Him for what passed. They did not see, but it was told to them.

Malbim: We do not understand the grandeur of Hashem Himself, only His deeds. Shevach is something that improved better than it was. The more that each generation looks into Hashem's deeds and secrets of nature, they recognize wondrous Chachmah that was hidden from those before them. They praise His deeds more than until now. Each generation sees that the previous generation did not understand His acts properly.

Malbim (4-5): Aside from praising natural acts which were fixed from the six days of creation, they will tell His Gevurah, what He does against the course of nature when He wants to overpower His enemies. Hagadah is a new matter.