You say old games bosses weren't very varied mechanically, yet RE4 is the same, you just point and shoot and it's only the way you dodge or do special attacks, both via QTEs that really makes any one different from another.
They all just running for distance and then standing to shoot until they near again.
El Gigante fr example - Just run/shooting until you get prompted for QTE with knife. Others you might QTE dodge too or instead.
The only really different one is Del Lago with the throwing.
I disagree with the assessment. Well, firstly I didn't agree with the original point the classic RE bosses didn't have any differences between them (as they did). But there is definitely a few fundamental differences between RE4 and the RE before that bosses:
01.) Weakpoints. In the old RE game due to aiming restraints, the weakpoints of enemies were downplayed immensely to make use of the games natural controls and auto-lock.
02.) Weapons. In the classic RE games, your arsenal was a lot more limited, which is a big point of the games. In RE4 you have a bigger selection of weapons with different pros and cons against different enemies, and those weapons have upgradable stats each. But upgrading weapons will take money from other potential items/upgrades.
03.) More varied behaviors. This is actually the fundamental point I want to make, but in the older RE's the bosses behaviors were a lot more limited. I won't say they were they were all the same, because they weren't. But the bosses in older RE games always would have a consistent exclusive to themselves that they would do, and many of them were to approach you. In RE4, bosses are programmed to dynamically respond to your positioning in an arena more and change their behavoir based on that.
There's some other minor differences, but I think that covers it. GIVEN, I didn't agree with the original argument you're arguing against in the first place.