I'm a Reception teacher (Inner London state school). We do a staggered start, taking 5 children a day for the first week (all half days), so all children are in by Friday. Then the whole class does full days from the following week.
I appreciate that it may seem inconvenient, but there are many, many good reasons for doing this.
Not all children have been in nursery. They may have a complete meltdown when left alone for the first time.
Roughly 40% of my class speak little or no English (as do their parents). They will need lots of additional support to settle in and understand even the most basic routines.
About 45% will have some kind of additional need. I say 'will have' because I've not met all of them yet. There are usually at least two children every year with substantial needs that we were unaware of. They will also need a lot of support. Imagine, for example, that five children in the class soil themselves regularly, or are sent to school in nappies, or have no sense of danger and cause harm to themselves or others. This is potentially manageable with a staggered start, but would be chaotic and potentially dangerous without.
School lunchtime can be incredibly stressful for new Reception children. Even sitting in the hall with lots of noisy older children is daunting. With five children every day, the TA and I can sit with them and make them feel more relaxed. (Bear in mind that some of them are still being fed by their parents, or might not be familiar with using cutlery).
If all children were neurotypical, I would still recommend a staggered start. It's a really big deal for them to start school and they should be supported to be comfortable and happy.
I can't answer the slightly sarcastic question about how it impacts learning outcomes per se, but can say confidently that none of my students were inhibited from reaching their Early Learning Goal because of this approach.
I hope that adds some useful perspective. Your own child might cope, which is great, but a significant number cannot.