The letter argues for the full implementation of the GDPR
Currently the differences between laws in EU member states mean thought has to be put into what needs to be done in different EU countries. With GDPR there is going to be greater parity between states, yes - I know it's not goint to exactly the same everywhere but a huge improvement on today when you, for example, compare Ireland and the UK. Ever had a chat with LinkedIn (located in Ireland) about your personal data expecting the same data protection rights as we have in the UK? I have and they don't exist (respectively).
If you want to have data moving freely between corporations in the EU then common rules seems to be a good idea. The GDPR will also make Europe the place to put your data if you want more than hollow words protecting your rights to this (your) data. Creating a strong regulatory framework for personal data is much the same as having strong regulations for accountancy and housing property ownership. Trustworthy regimes promote commercial activity. This will promote the growth of the digital economy in the EU, not hinder it.
BTW - Safe Harbour was not the predecessor of the GDPR. Directive EC/95/46 was. Enacted as, for example, the Data Protection Act in UK, Wet Bescherming Persongegevens (WBP) in NL and the "Pointless Piece of Paper" (PPP) in Ireland. Safe harbour was a self-certification scheme run by the US Department of Commerce so that US entities could pretend to provide the same level of personal data protection as we have in the EU. It was, frankly, bollocks from day one and the Snowdon revelations had nothing to do with that. Killing it has been a step forward.