Regardless of how the mining scarcity is implemented, that really won't do much right now when talking about the volatility. Bitcoin is volatile simply because it's being widely used as a speculative investment, and not really as a store of value. A good percentage of all the bitcoin holders bought bitcoin for the sole reason of making profit, and nothing else; hence we have these huge hype cycles. Bitcoin could already be at 21-million mined BTC right now and bitcoin would still be very volatile.
I think your "solution" to reduce price volatility and increase price would only work if we assume that everyone simply just holds; which is definitely not the case.
Bitcoin's 4-year block-reward halving schedule arguably exacerbates wild price fluctuations, instead of a steady rise.
?

?