At many D3 schools, there will be a couple of spots left over for walk-ons. It’d be unusual for every coach to have enough “picks” to fill all of them. But sometimes that happens. I have heard of kids who ended up playing club soccer their freshman year (because there were no spots) and got a roster spot sophomore year.
If a coach promised recruits a certain amount of playing time (in their first year), that may impact the walk-on’s ability to get playing time freshman year, but it doesn’t mean he won’t play or won’t get more time in later years.
This depends on sport, school, and kid. And often on the year and where a particular team’s roster stands. As you’ve no doubt seen discussed on these boards, many athletes do not play all 4 years. There are tons of reasons for that.
I think you also have to keep in mind that not every kid is recruitable at every school, so there’s a possibility that a kid ends up at a school that wouldn’t have had any interest even if he hadn’t applied to MIT - level of play is higher, style of play is different, lots of depth in kid’s position.