Another reason I like NOT doing sit ups is it helps reinforce the movement pattern of LOCKING in the midsection while your hips, legs, or shoulders move weight to make you stronger and healthier.
If you think about a sit up, the spine crunches forward. In what lift are you ever crunching your spine forward? Maybe a turkish get up, but if you are crunching your spine forward, you're probably not doing the move right.
In all the moves that help get you strong, increase mobility, increase bone density, and improve your body composition, I can't think of a single one where forward movement of the spine helps the movement.
Most of the time, you are actually fighting the forward crunch of the spine to stay upright. Meaning, your spinal erectors are pulling backward and your abs are actually pressing backward and staying EXTENDED as much as possible.
That's why I like doing moves like the birddog, the deadbug, and cat cow to encourage the spinal erectors and the posterior chain (ie, the back half of your body) to work more.
I'd like to introduce to you the prone hip extension. This is a phenomenal move that keeps your spine neutral while you squeeze through your glutes to bring your hip to full extension (rather than move through your lower back to get your hips to full extension).
In this video, my client JJ is hanging her legs off the bench to get full hip movement forward and backward while keep her spine neutral.
This is an excellent position. Her low back is still getting a little work, but it is the big muscles of the low back that are working, not the tiny muscles of the lumbar spine or the quadratus lumborum (ie, the muscle that connects your ribs to your hips) that is doing the work.
Another version here of me doing this with my legs straight. This puts a little more leverage on the hip joint and it is tempting to really squeeze this through the low back. Fight that temptation by keeping the movement in your hip through out. This is one of the most underrated glute exercises out there.
To reinforce how the spine fights OPPOSITE of sit ups during lifts, here are some videos showing as such.