Originally Posted by
Demolitia
Precisely, the political declaration is just a non binding declaration of intent on what the future, permanent trade deal should be. It is very vague and open ended, intentionally.
The withdrawal agreement (the deal) in its current shape ensures that there is no border in ireland, that we keep trading on current terms, that you don't pay EU membership anymore and that you can start working on trade deals with other countries.
A customs union alone would no achieve that. While I'm pretty sure both sides would stick to the same standards, at least in the immediate future, we need some legal guarantees and mechanisms to ensure that consumers know what they are buying. There is not trusting each other on this. Right now we have common treaties, institutions, regulators and arbiters that decide which rules we all stick to. Once you leave, you will not be bound by them if you are just in a customs union, and we will not trust you. We will put in border checks to protect local businesses and consumers.
You can join the single market and pay a fee for the regulatory work the EU does, but you will have to accept the ECJ's authority if there are any disputes.