when I played age 3, cav had nothing to do against my amassed artillery. I played all campaigns where it was possible using only gunpowder because it was that easy.
I still say its a failure by MS Standpoint because THEY DIDNT DO ANOTHER ONE. Until now.
Not calling the campaign a failure and yes, you are right, they did believe it back in the time. Which makes it a bit more fitting to me, but I still like nowadays perspecitive more.
also its natural progression if you change some things, but the core principles were changed. many of them. Many of them which were fun and part of strategies 
edit:
I checked, it seems the game was selling well and receiving awards and good critiques. I however only remember the bad ones, especially the hardcore grind. Maybe I am quite biased there. But also MS cant have seen it as huge success either , as mentioned, because we get age 4 now, 14 years or so later