@Carcer The way I've phrased it before is "assume a rule has a reason to exist". There are rules that give certain classes the ability to cast spells which would have no reason to exist if anybody could just do it. There are no rules giving certain classes the ability to fry eggs, so you won't break anything if you agree that just anybody can do it.
@Glazius yeah, that's kind of the way you have to take it, but the problem is that then you need a pretty complete knowledge of all the game's rules to know whether or not a particular kind of action might be covered by a rule somewhere
and then mechanic creep in splatbooks means that things that were not originally covered by any rules anywhere may subsequently become covered by rules published later
"Can I take some common household chemicals and make a pipe bomb?" "Well, there's no rules against it and there's no rules specifically granting that ability to anyone else, so sure!"
No. Fighter is an exception to the rule that you cant make more than two attacks with Extra Attack, not an exception to the rule that multiple extra attacks dont add together.
> If you gain the Extra Attack class feature from more than one class, the features don't add together.
This is one self contained rule
> You can't make more than two attacks with this feature
I think what they meant was "if you have extra attack from multiple sources, like fighter, paladin, and ranger, each instance does not give you an additional attack"
so a 5 pal, 5 ranger, 5 fighter, wouldn't have 4 attacks
> You can attack twice, instead of once, whenever you take the Attack action on your turn. Moreover, you can cast one of your cantrips in place of one of those attacks.
Bladesinger allows you to replace one of your two attacks. If you're making three attacks, it isn't "those attacks", it is attacks from a completely different class feature.
So bladesinger and fighter extra attack both have the exact same "you can attack twice" text minus the level difference and "Starting" vs "Beginning" (synonymous so not important)
Fighter further has the bonus of gaining a third attack when you hit fighter 11, while bladesinger has the extra of being able to swap one of your extra attacks for a cantrip
To your credit, before the bladesinger errata, "features" did just mean "attacks", because none of the extra attack features did anything but give more attacks.