The block subsidy will be determined by the free market once inflation is no longer relevant, iff the block size limit is dropped. Even Bitcoin inflation itself in a sense may one day be determined by the free market, if we start seeing investment assets quoted in Bitcoin being traded with high liquidity: such highly-liquid BTC-quoted assets would end up being used in trades, and would become a flexible monetary aggregate. Fractional reserves is not the only way to do it.

Concerning the time between blocks, there have been proposals of ways to make such parameter fluctuate according to supply and demand. I think it was Meni Rosomething, IIRC, who came up once with such ideas. Although potentially feasible, that's a technical risk that might not be worthy taking. Perhaps some alternative chain will try it one day, and if it really shows itself worthwhile as a feature, people might consider it for Bitcoin, why not. I'm just not sure it's that important, 10 min seems to be fine enough.