"The decision of these changes has been made with careful consideration as we have not adjusted the miles in our award charts for almost fifteen years ago since the beginning of our program."

This is a false justification. Any increased ticket prices will necessarily be reflected in the cost to the passenger in accruing award miles. So if (and this is a purely hypotethetical example) BKK-SYD cost $1000 10 years ago, and costs $2000 today, it will have cost the passenger twice as much to accrue those 4674 miles today as it did 10 years ago.

Because award mileage accruals already reflect changes in ticket pricing, raising award redemption rates is just a program devaluation, pure and simple.