I think what gets to people is firstly, it's not the same price. When you apply the tax from 2018 vs now, the car is still cheaper. And yes, of course many people were acting out of 'greed'; if you see a chance to legally benefit from something, you can't blame people for wanting to do it. It's not called an 'incentive' for nothing.
I also think that they need to be careful that this doesn't actually cause deterrence for their next launch. The last thing they want is everyone to be hesitant to run out and purchase the Model Y straight off the bat for fear that it'll be cheaper in a year's time. And yes, there will always be people who will pay premium to be the first to get something new, but it's not an iPhone and we're talking thousands of dollars. I guarantee in hindsight many people who bought the M3 in 2018 would have chosen to wait if they'd known what would eventuate.