I don't think anyone is arguing that one single idea is better than a fully completed idea group. Defensive gives other valuable bonuses -- the morale is just the most obvious one, and I believe it's better than any single idea in the Offensive line.
Morale << forced march (picking where you engage and/or catching people out is that valuable). However, it's probably better than most of the offensive tree other than that. The discipline is being underestimated here; casualties literally cause morale damage too. Also, in chasing a shattered retreat vs a big nation you've no guarantee you'll be able to win when you get there. Take for example big wars like Horde vs temple faction Ming or human Burgundy vs France. Those aren't situations where you normally want to chase too far into the territory straight away, but if you know you just shattered their 1 stack, FM, not defensive, is what lets you wipe it.
Inflicting more casualties also means you take fewer, so in any battle you actually win, 10% discipline > the morale in defensive unless a follow-up stack reaches you before you recover it.
Defensive relies a lot more on attrition + manpower, and its reinforce speed bolsters that angle. Unfortunately, by the time you get the base attrition rate booster it's too late to make much use of it. While the tradition IS nice for defensive, and synergizes well, the kit just doesn't add up to what you get from forced march + solid other benefits.
However, I can't emphasize enough that until late-game, none of this stuff beats tech. Western goes military early because if they don't, they can wind up "ahead of years" so far that they spam buildings or get stuck on 999. Nations like Indian, Chinese, and Nomad shouldn't even consider a military idea group until 1600ish. You *can* get "ahead of years" in the Indian/Chinese groups, but it's late that it happens and only if you don't take ideas. That's worth its weight in gold; those groups can field 50% or more pips than Muslim from tech 12-20, and the Indian 12 is 20 pip, 1 less than Europe's at that time. Europe can still beat you # for #, but if you're even on military tech, you can swarm them + crush them easily if you have full combat width and they don't, and you'll win even a small lead in #'s if you have the cash for chambered demi cannons.
When you can use someone like Hindustan to wreck Muscovy/Russia and take Siberia, you've some oomph