We are in a similar boat, and now DS is nearly 6 and I am in my 40s I have come to terms with being a family of three. It is hard.
But: no sibling squabbles (this honestly seems like a major win listening to my friends with two primary-aged kids).
DS gets to do any and all activities he likes - no issues with clashes and having to compromise, no major financial constraints.
Basically he hardly ever needs to compromise on anything - if he wants to go to the park we can, if he doesn’t want to he doesn’t have to. No keeping siblings entertained or dragging him out to Barbie World for a sibling’s benefit (obviously he has to compromise for our needs/work/school etc, but family time can be led by him). Huge amounts of 1:1 parental attention, help with school work, enrichment activities.
No siblings annoying him and his friends on play dates.
We only need one room on holiday (most hotels will do an extra bed or 3 bedded family room, not many can accommodate 4 beds).
It’s fine for one parent to have some alone time/go out for the day as one parent can easily manage one child. Leaving one parent with two kids is a much bigger ask.
Bedtimes and bathtimes are fine. No queues, no winding each other up, no fighting over which parent reads whose story tonight. Just calm bath and bed.
We’ll be able to support him at Uni in a way we couldn’t afford to support two. He’ll also eventually hopefully inherit quite well.