If bitcoin price is very high in future, there won't be any problem for miners because they still earn same amount of money (in fiat).
Also, ASIC or mining device will be more efficient than now in future & electricity price could be lower if scientist can find better way to produce electricity for lower price.
So, i think there won't be any problem if those condition are fulfilled.
Do you mean the fees amount in BTC will not change? But if some guy bought 0.01
BTC for 100$ ("bitcoin price is very high in future") and he have to pay 0.0001
BTC (one $) per each transaction? What if he wants to buy some good that costs 1-2$? 50% fee it is very bad. If the whole world will use Bitcoin miners will do all that I wrote.