Please tell me what do you think will happen IF If in 8 years we have negative growth but a 2mb or even 8mb blocksize?
Nothing catostrophic would happen, less would be payed for security since obviously the world does value Bitcoin as much as I thought it would under that scenario. This is not catastrophic and I do not think that having more expensive fees would help this situation at all.
Will transaction fees alone be enough to secure the network?
Yes it will after a few decades or sooner depending on the rate of adoption.
Will miners process torrents of spam for dust?
They would not, they would simply choose not to include those transactions.
Will our network be stronger or weaker?
Stronger, I think that realistically there really only is one way forward, if you do not think this will succeed that is your prerogative, I do still believe in the original vision of Bitcoin.
Are you willing to risk everything on unfounded assumption will continue to grow?
Yes, since if it does not grow it will have failed anyway. Bitcoins security relies on this assumption.
Im sure that in 20 years there will either be very large transaction volume or no volume.