If Bitcoin can survive and even prosper for 100 to 150 years with the current set of "rules" what would make you think it needs changing? If the "flaw" is fatal as you indicate it will likely result in the destruction of Bitcoin long before your implemented change would take effect. I noticed initially it was 50 years, then 50 to 100 years, then 100 to 150 years. The longer the time horizon the more dubious the change. Lets say over the next 150 years 0.2% of the coins on a nominal basis are irrecoverably lost. 150 years from one the money supply would be ~0.74 million BTC. If 200K BTC were lost in the first year it would be introducing >30% annual inflation to the system. Lets also point out that the system will have "adapted" to the slow attrition of coins (which is meaningless as "money" is simply an accounting system). The worst possible thing 150 years from now would be to suddenly dump a fortune of new coins into a system which has long since adapted over the decades to a slowly shrinking monetary base.
There is no validity to the claim that the money supply must remain perfectly fixed at 21 million BTC. The goal of the fixed minting schedule is to ensure that inflation and deflation can't be MANIPULATED for the benefit of a few at the expense of the many (like fiat systems do). Satoshi could have decided to have minting go on forever at 1 BTC per block. The small amount of inflation that would occur many many many subsidy havings from now would be negligible and wouldn't be subject to manipulaiton.
Small rate of inflation ~= perfectly fixed money supply (completely unatainable even under your proposal) ~= small rate of deflation. Inflation and deflation aren't a problem as long as they are low and without volatility.