We don't update software that promises "performance enhancements and optimizations for older devices" then get 98% of the old system. The minimum should be 100% what the system already offered, then go from there.
In old non agile days, companies shipped when system reached "maturity", not sooner as treating their customers as beta testers. This is the BS that started with "free services" and "ship it faster" mentality in Silicon Valley and sadly Apple seems to have fell victim for it too.
Customers don't buy products based on future possible optimizations, but how it performs NOW. If it doesn't perform NOW how it was yesterday following an update, they will complain as the product wasn't FREE and it's their right to do so.