Before I had kids, or when my youngest was still a baby/toddler I thought pre-school was 'mandatory'.
I grew up starting pre-school close to 3, went half days for the first years and full school days in the 2d and 3d, and I've always loved it, have very fond memories, too. It's the habit in my country of origin to send children to pre-school from the age of 2,5-3, starting to be popular somewhere in the seventies? It was just a normal concept to me.
I even remember thinking about a very energetic/spirited neighbourhood child where I now live of him 'needing to go to school' when he was about 4 and his parents hadn't sent him 'yet'. I now know I was wrong in making this assumption.
When I had a child of my own I thought of not sending him because it would mean private pre-school, and in my own country I went to fee of charge state pre-school so couldn't imagine paying that much money for sending a child to something I've had 'for free' (and state pre-just started around here and requires a fee just as private, may it be cheaper).
Ds1 at 2,5 started to first inquire about 'going to school'. He was the only one of his age in our neighbourhood starting to play outdoors more often and his neighbourhood friends, who were 3-4y older started kindergarten/primary school by then. Him being extraverted, sociable and loving other children and asking 'when he could go to chool' contributed to our sending him to pre-school at 3,5. He was involved in finding and choosing the right school for him, the search itself was a way to learn to understand his need and level of readiness to go. He was so excited. Half days would be what suited him so full days with required napping which he didn't need were out of the question. We didn't need a daycare, he just wanted the play and activities and being around peers.
At the same time, my need to spent more quality one on one time with ds2 was met, too.
Now I see that ds1 is often tired because of school. Sometimes he's a bit bored. But he still loves to go in general. He usually doesn't mind going to school in the mornings. He's just so used to it.
I've always assumed that ds2 would start to go at the same age (3,5). He's now 3 and I'm just not so sure about this anymore. He's a very different personality than his brother, he is high needs and a very spirited/explosive child. He has more peers around here for him to play with. He doesn't like a fully controlling structure. He needs one on one time, and I need this with him, too. He likes to be outdoors with me. The same pre-school as ds1 may not even be such a great idea because of their sibling rivalry and their obvious need to spend time away from each other. He sometimes likes to enter his brother's school but I know it's only for playing with the toys there. Ds2 is not out of diapers at the moment and we do CLPL (lol: child lead potty learning) and he's not ready for this yet. So no, now I'm not so convinced ds2, or therefore any child NEEDS pre-school at all, or at a particular age. For some it could be beneficial, for others it would be better to not go.
Ds1 could've done equally well without pre-school, I'm sure. But he WANTED to go. And I will look at ds2 closely for what he wants and needs as an individual. If we (him included) would believe it beneficial for him, and us, to go to pre-school, than he'll go. And then we'll find an arrangement that suits him best. And suppose it wouldn't work out, I would be ready to have him back home with me, too.
In my coi, pre-school is seen as a nescessity by government and most parents/grand-parents (working parents, and also most sahp(may there not be so many these days)) as a general NEED for children 2,5-6 to attend pre-school. Not always as the need for the individual child though, but based on other, more generalised beliefs/reasons (eg independency, daycare situation, learning, socialising, learning school discipline).
I now changed my perspective and do not believe it is a MUST anymore, just an option.
My family is convinced it is a must though, so we already got some comments on ds 'needing' to go to school by now (and his almost same age niece is already going for 4m now), and am sure there will be more comments/urges, especially if we decide it's not time/suitable/needed for our son to go, and definitely if we would not send him to school until 4, 5 or 6. But we're not sure about what he'll need in the near future anyway.
Dhs family doesn't see pre-school as a must at all since in their days/life/culture/country it just didnt exist and and even in their present culture/lifestyle it is not a must at all, pre-school is also something very new in the society I now live in, and mostly just in the major cities. However, views on early 'education' (and daycare, and wohms) have been drastically changing over the past decade, and also government's approach, so now pre-school seems to become a 'must' for many of the 'modern generation', mostly in the urbanised areas still.