I think there are already some Euro countries using a “forever” stamp.
What’s your 2 cents on the forever stamp idea?

Article
Postal officials pitched the idea of creating a "forever stamp" that would be good for sending first-class mail no matter how much -- or how often -- the cost of a postage stamp goes up. The announcement came on the same day that the Postal Service said it would seek to raise the price of a first-class stamp for the second consecutive year.

The forever stamp, which would cost the same as a first-class stamp, would provide a hedge against future postal rate increases and end the search for 2- or 3-cent stamps that usually follows a price increase. The stamps could pose unusual challenges for the Postal Service, however, and officials say many details still have to be worked out.