There’s nothing preventing Bitcoin miners from modifying the Bitcoin supply algorithm and inflation rate, except for their collective unwillingness to do so.
Bitcoin block rewards go down over time in Bitcoin terms—but so far the long term trend has been that the rewards have gone up in USD terms. We have never seen a sustained, long-term decline in the block reward in USD terms.
When we have seen short-term declines in the USD-denominated block reward, the hash rate has also declined—meaning a good number of miners have stopped mining.
What happens if the price of Bitcoin stagnates in the long term? Will miners still mine when the block reward is slashed again? Or will they decide to modify the algorithm to ensure that they remain profitable?