I disagree.
A gap is usually 6 to get one away.
If you have a 777 following a 777 with a 747 to depart in between the gap required is 6.
if you have a 738 followed by a 319 with a 738 to go in between the gap is still 6.
Defaulting to the lowest common denominator spacing (which is based on the worst case traffic mix) does not lead to an efficient system.
There are savings to be had by having tailor made gaps calculated to suit each aircraft type rather than default to 6 miles, which is enough to get a heavy away between two heavies.
Also 737s could do 170kts to 5 and A319/320 could do 160 to 5, increasing stable approaches and reducing go arounds.
Wouldn’t it be better to have a systematic spacing delivery where landing clearance is consistently given at, say 200’ than varying between 50’ and 500’ ?