Let's run the default through a few scenarios.

You're a 5'8" guy who bikes to work. Regular fit under a jacket works. Oversized would flap and snag. Default holds.

You're a 5'2" woman who loves oversized hoodies. An oversized tee might overwhelm you. A regular fit with a slightly dropped shoulder (regular-oversized) could be the sweet spot. Default might be too strict.

You're in a hot climate and live in cotton tees. Both fits can work, but oversized in a light fabric is breezier. Regular is more forgiving for sweat stains. Default still holds.

You're a creative professional in a casual office. Regular fit tees in solid colors look polished. Oversized graphic tees might be too casual. Default wins.

So the rebuild isn't rigid. It's a starting point. If you're still torn, buy one of each in a neutral color and test them for a week. See which you reach for more. That's the only real test.