My DD was 4.5 when she started and it was the right age for her, physically looked ready (v tall - needed 6yr old clothes), mature and happy to sit colouring etc.
If we'd had an extra year or two at home I'd have continued in the same vein - nursery 2 or 3 days, music group, dancing classes, swimming lessons etc. She was beginning to read and no doubt would have been reading before entering school at 6 or 7.
In effect i'd have been homeschooling her - mainly through play - boardgames, reading street signs on the way to the park, workbooks as DD enjoys them etc.
The big market for preschool activities would expand - DD did Kindermusic which was an american franchise. It had a syllabus for 5/6 yr olds as US children start school later.
I live in a very deprived area - one of the trial areas for govt funded nursery at two. I know the majority of children wouldn't have the same experiences.
The gulf on starting school at 7 would be huge.
My understanding is countries which start later also have good nursery/kindergarten provision. Children don't do from home to a reception equivalent age 7. They are expected to do alot of formal work/homework when they do start.