There was no real way to monetise it though back then. This was before The Sims developed the business strategy of lots of small expansions - and even that series had sequels pretty often - and microtransactions, DLC, etc. were all years away. There were very few games which were supported long-term during this period, and most of them were MMOs with a subscription model.
Also, it's easy to say in hindsight AOE3 was not that great a product, but at the time I'm sure they expected it to do just as well as AOE2.
I disagree!
I can think of lots of franchises that had huge success releasing new games that were basically the same base game but always came with little (or big) improvements: Tomb Raider, FIFA, Broken Sword, etc (I'm pretty sure there are dozens more, but I'm a bit in a hurry right now lol)
AoE 2 and AoM were however 2 absolutely different games, much more different than AoE 1 and 2.
Also, yes, hindsight is 20/20, but that's exactly what I'm saying: learn from your past mistakes and don't repeat them in the future.
Meaning, don't abandon AoE2 when AoE4 comes out.
Doing so would be making the same mistake twice.