While I understand that this is the current plan, what is the guarantee that this plan will not be changed? Is this a technical limit, or a social one?

The "plan" is implemented through the bitcoin protocol and software.

Any change would need 50% + 1 of the bitcoin network to follow the new rules.

An effort to change the software to "mint" a greater amount will be countered by those who already hold bitcoins, and can add mining capacity to resist the change.

Another implementation could be launched with a different limit, but that would be a completely separate, new currency.