We kept Nashorn for 4.0 because we wanted to be on the safe side — if something does not work with JS Scripting, one could go back to Nashorn.
I think meanwhile both JS Scripting and Blockly on JS Scripting have been proven to be reliable, so from I’m fine with dropping Blockly for Nashorn with 4.3. This will definitely reduce the amount of Blockly code needed 
The only advantage Nashorn HAD IMO was its faster „compilation“ times on 32-bit, but with [rules] Add support for pre-compilation of conditions and actions by florian-h05 · Pull Request #4289 · openhab/openhab-core · GitHub this advantage is gone/the slower compilation times of JS Scripting are mitigated.