I don't see any downside to this at all.
Currently we have a bizzare mixture of CAS bases in altitude and flight level. In some cases you have e.g. 5500ft and FL055 next to each other. Anybody looking to reduce CAS busts will tear their hair out right there.
There is no pilot-procedural downside because low airways IFR traffic (below 18k) will almost always be in CAS and then ATC pass you the QNH along with the altitude to fly. You don't dial up anybody's ATIS to get the QNH. It is only OCAS that the pilot will need to get the QNH and that is what he does at present anyway (or should do).
The alignment with US practice is a bonus. A lot of foreign pilots trained in the USA and they find Europe's airspace, with the Eurocontrol computer with its perverse rules, totally baffling, and the 18k transition will reduce the baffle factor a little bit
Pulling the plug on the whole of Eurocontrol would be the icing on the cake - I have just finally managed to file an IFR route for tomorrow (with DCT hacks) which defeated all the software tools, including Eurocontrol's own "route suggest" facility
