In reply to gooberman-hill:
For existing policies: I would expect them to remain valid but you would need to read the policy in question and/or contact the insurer to be sure. My understanding of the BMC one for example is that you aren't required to use an EHIC but the excess is removed if you do.
In the future: I would imagine the EHIC arrangement will continue in some form eventually but even if it doesn't I think a European policy would still be cheaper than a world one - the costs of healthcare, repatriation and rescue are still likely to be cheaper (or at least easier to quantify) Europe-wide rather than worldwide.