My family used to have the view that keeping cats indoors was cruel until one of our indoor/outdoor cats came back after being attacked by some unknown animal in what seemed like a safe suburban area. He was fully vaccinated and recovered mostly without incident, but he had to have a surgery and spend extensive time in the Cone of Shame with limited movement so he could recover.

And once we started keeping the cats indoors? It was unbelievable how much nature outside recovered. It was more than just songbirds everywhere (though we did have plenty of those). We saw hawks and owls. Woodchucks and deer would wander in (yards weren't really fenced at this area... it was more on the rural side of suburban). Foxes and rabbits and wild turkeys. I once saw pawprints in the snow that I think were from a stoat. We could not believe how visible a difference just keeping the cats inside made.

And honestly, the cats didn't really miss being outside after a week or so adjustment period. Once, a year or so in, Mom accidentally left the door to the outside open, and one of our cats just got this resigned look like "oh, I guess I have to go patrol the area, huh."

Now, this was a two-story house with like five bedrooms and a basement, so it was more territory than, say, an apartment, which can make a difference. We also had two cats to keep each other company, so the house being empty during the work/school day wasn't as big a deal. Space concerns are a bit tougher to address (though I will recommend catification, catios, and leash training for cats to do supervised outdoor walks), but if the concern is a cat getting lonely during the work day, then I'm just saying that two cats are not appreciably harder to take care of than one and after the introduction period they can do wonders for keeping each other entertained.