Line trigger is an important feature, don't muck with it.
Share with us your external trigger needs and there may be another method to achieve the result you need.
Why? Who needs Line trigger nowadays?
Anybody working with mains frequency referenced equipment.
Anybody examining ripple in a linear PSU.
Line Trigger FORCES triggering at line frequency, very useful on occasions.
You may or may not be aware many scopes these days can operate from supplies ranging from ~45 to 400 Hz, wouldn't Line triggering be useful over that range.
Fact is it's all internal, cost little to implement and arguably more useful than Ext trigger, however IMHO both are a necessity in a decent scope.
You might ask the same question as why various video trigger modes are included in scopes these days with the prevalence of new formats like HDMI.
The only reason to omit it is cost.
This is a bit of a tautology,
Put yet another way, think of a 2ch scope with ext trigger as a "3 channel scope with a crippled, trigger-only third channel". Now a non-crippled, 4 channel scope doesn't sound so unpalatable.
Isn't all this JUST another selection criteria one must apply to a prospective new scope.
Years ago before LA's a 4ch scope with Ext triggering WAS your LA and for those that can't justify a separate LA unit these days, a scope with Ext trigger is an attactive option.
Used that senario several times myself.