If it was a one off like PP it would bring a lot of people back into the network.
Agreed, but would that be a viable business model in the long term? Without a regular income stream, how would Airnav pay the monthly server costs, bandwidth, etc.
And remember, whilst PP is a one-off fee, that doesn't actually buy you the sharing service, note the COAA disclaimer that "The Internet sharing and multilateration features are to be regarded as experimental and
may be withdrawn at any time." You could pay €25 for PP now, and find the sharing service removed the next day.