Do you mean that with the small amount of rewards received by miners, it reduces miners' interest in mining activities? We know that all jobs should be profitable in order to keep running... if the BTC reward for miners is no longer in accordance with the costs incurred, it could be a sign of collapse...
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Technically speaking, collapse is unlikely, as there will always be *some* reward for mining, even if the block subsidy goes to zero, there would still be transaction fees left.
The system is designed so it can scale up or down when needed (i.e. difficulty adjustments). The problem is that the lesser the mining reward, the lesser the hash power, meaning less network security. Not to mention that if BTC's price was to drop very significantly with no sign of revival, some miners could decide that instead of mining it's more profitable for them to attack the network, i.e. by performing double-spends etc, which, depending on jurisdiction, could even be perfectly legal.