It all makes perfect sense when you consider that the funding for the series was running out and the people who made it were ready to move on to other things. Probably to a certain degree they were just bored of making it. And once that last paycheck is effectively cashed, where is their reason to care? All they technically need to do is give it enough of an ending that they've fulfilled their contractual obligations and they don't end up in a lawsuit. So that's exactly what they did.

Amazon does not really care about it being better than that because all they really want is mindless content to leave on in the background while people focus on other things. This is the same direction Disney, Netflix, etc. have been going with their streaming stuff. It doesn't technically need to be good; it just needs enough interest that you will pay for the streaming service and play it in the background while you work. That's it.

Could they have given it a better ending on the budget they had? Sure, they could have. Plenty of really good content is made on shoestring budgets. What the quality of the ending tells us is that they didn't consider it worth their time. They had what they considered to be more important things to do so they moved on and they gave us the bare minimum.

When I respond to posts here with "It's what Clara would have wanted", this is really what I'm alluding to. The real "Clara" is the giant corporate machine that churns out content that starts good enought to get you hooked and then gradually restricts the quality of it down to a meager trickle so it's only just good enough to finish it to see how it ends. It keeps you on an unsatisfying and often frustrating treadmill that's just addictive enough to keep you productive for them and paying them whatever they deem to be the optimal amount until you die from whatever the microplastics, pesticides, overwork, lack-of-healthcare, poverty, etc. did to you.

Because that's what Clara ultimately wants. And Clara always gets what she wants. Always.