There are downsides.
I have a really great rate on my mortgage, but our house is super expensive and small for our family… but now we can’t afford to move.
If we moved to a new house, we would have to pay off this great mortgage and get a new one, at a much higher interest rate. Even if we found a house that cost the exact same as ours, the monthly payment would be 50% higher, because current interest rates are more than twice what we have. We are locked into our house.
This is some drawback. "I have access to the same bad alternatives as everyone else."
Do 30-year mortgages make the other houses more expensive somehow? It sounds like you got a good deal and any change would be worse than the good deal you got. I'd appreciate it if I was you.
Edit: unless you mean that the downside of 30-year mortgages is you hardly get to pay off the principal in the first several years and don't build much equity maybe? That's more a "long mortgages" thing.
If you were looking to buy a house right now, you'd be looking at a bunch of options that are worse than what you currently have. You experience this as lock-in but in reality the "problem" is just that you have something significantly better than anything you (or others) can find on the market right now. And of course that's actually a fantastic boon.