They don't have to justify anything - people are spoiled now. They operated for 30 years with no FP on most attractions. It did not make the company money like they expected, so now they are going to switch to a system which is guaranteed to make them money.
Also, yes, in your example, Space Mountain line would have been an hour without FP - so maybe you saved 45 minutes. However, the minute you step into any line which you don't have a FP for, you are giving back that time to Disney. It's like sitting down at a blackjack table. You might win five hands in a row, but the longer you stay, the more you'll give back to the house.