Unfortunately, in most cases, they are the same pump. The consumer just pays up front for the free replacements down the road. The mfgr is betting on the consumer to not take the pump back each time and most won't !
You just proved my point...lol
What incentive does a manufacturer have to build higher quality equipment if consumers buy the less expense model nine times out of ten anyhow?
In the business world warranties are treated as liabilities. Requires a percentage of each sale be put in escrow (warranty reserve) for the duration and it also affects cash flow, the life blood of a company. Given possible legal implications, actual warranty costs can far exceed that of the purchase price.
As such, nobody in their right mind, well maybe the guy working out of his garage, is going to increase their exposure by offering a warranty based on shipping replacements for the duration.....that's not a viable business plan.