Belts are aids in that they allow you to literally flex your stomach muscles and valsalva to a great degree - they provide no intrinsic support, instead providing something to brace your own muscles against. This is literally the argument one makes for core work! I also agree - it's important to learn to be able to squat without a belt, and I largely will with the exception of my heaviest sets of the day. It helps you appreciate the assistance and work on your own unaided valsalva.
If you do slip into this catastrophic scenario you're visualizing, being able to hold a static plank for 3 minutes isn't going to help that - you haven't trained the muscle at all to do what it needs to do at that moment - stop being static and move. Having the sheer grunt power and experience of having hundreds of lbs on your back for years is what's going to save you, and I'd argue that there's plenty of muscle groups that you've built up over the years to keep you safe in just such a circumstance - your spine isn't floating in a vat supported only by abs. I've dumped the bar, failed lifts, things have slipped, but I've never slipped a disc because I am strong enough to survive the load.
Do accidents happen and people get hurt? Absolutely, but both runners and old ladies slip discs too.
In a perfect world, this is true - and I wish you continued good luck and health.
However, you should reflect on the mechanical advantages and levers that your body is capable of producing as it curves and flexes around a particular joint ...
500 lbs distributed nicely and evenly across all of your vertebrae works quite nicely ... but all you need is a little twist ... a little hiccup ... just two or three vertebrae coming into line instead of a gentle curve ... and you suddenly have multi-hundreds of pounds on one little lever.
Not to spoil it for you, but the squishy little disc loses.
I am a huge proponent of back squats without aids - like belts - for the purpose of developing natural core strength like what you have experienced, but it is very, very cheap insurance to work your abs/back/sides in addition.