Brexit takes ages, but not decades.

(And yes, the original text is decades old and it's quite obvious that the negotiation team stldecided to keep the existing process and therefore take the same legal framework)

Original text being collection of decades of individual collective works is key here. If we update this part to modern standards, why wouldn't we update all of the others too? And how long that process might take...

The point where is: The brexit agreement is not the forward looking thing, it is taking the status quo and extending it. The EU can go and improve it (if they see need) but requiring something differently/newer in Brexit negotiations causes more trouble. It is a complex transition already and brexit negotiators don't have a mandate to renegotiate EU treaties (which would be the consequence)