There is no point to run full node when network doesn't scale.
More nodes don't make transactions faster or cheaper, but the validation mechanism more decentralized.
If you don't care about decentralization you can just use Visa. Faster than any cryptocurrency. Bitcoin exists for other reasons.
Ask yourself,
Validation only occurs when the # of confirmations increases,
Confirmations only increase when a New Block is added,
Therefore non-mining node don't validate anything ,and are irrelevant to network security.
Non-Mining nodes could all turn off right now, and no one would care.