This isn't going to be a "gave it a tug, didn't open, give up and move on" situation.
And when it does break, you've got people in the aisle wanting their carry-ons - which may include medicine or other essential items - so you need to train the cabin crew on how to manage that situation.
If power goes out in-flight, which does happen, do the storage bins stay locked, or do they fail-safe to unlocked? Is there a manual override for when that medicine is really needed?
Consider Uruguayan Air Force Flight 571, "the Miracle of the Andes", where the plane crashed on a glacier in the Andes. The used what was in the luggage to rescue themself. Were it locked away, things would have been tougher for them.
Figuring out how to handle low-probability situations is hard.
1) Checked baggage in the baggage compartment is packed more efficiently than storing in the overhead compartment because the greater volume makes it easier to fill holes.
2) Larger planes save money by loading checked luggage into containers, and loading the entire container in one go.
3) Only small bags can be stored overhead, so this scheme requires different handling for small and large bags, which is more expensive.
4) Using carry-on baggage compartments means baggage handling staff must be in the passenger cabin to load and unload. They will likely block passengers and increase loading time, which costs money. If the baggage is loaded before passengers board, then the ground time and therefore operational costs increase because currently the baggage is loaded in parallel to passenger boarding.
5) It increases passenger dissatisfaction to have their carry-on luggage further away, which encourages people to switch to another carrier.
6) How much extra space do you think there is? I've been on many flights where people had to gate-check their luggage because the cabin ran out of space. Think of the cost if 20 bags had to be gate-checked into the baggage hold because 20 bags had been moved to the carry-on section. (And airlines don't know precisely how much carry-on baggage there will be, because people can buy things after check-in.)