I generally agree with you, but to be fair to the complainers, what sucks is that Docker didn't make it clear up front that it should be part of your accounting. I don't know if they always intended to monetize this way (if so, we'd call that a bait and switch) or if they sincerely had other plans that just didn't pan out, but either way the problem is the same: There's a trend across all of technology of giving your stuff away for free until you become the obvious choice for everything, then suddenly altering the deal and raising prices.
That kind of behavior has in the past been deemed anticompetitive and outlawed, because it prevents fair competition between solutions on their merits and turns it into a competition for who has the deepest war chest to spend on customer-acquisition-by-free-stuff.
At one point Docker probably may have had an authentic intent for a free service, but costs along the way changed the reality of operations and long-term cash flow and success of the business required making changes. Maybe the cash saved from bandwidth is what makes the next project possible that helps them grow the bottom line.
Further what was once a positive value proposition 18 months ago can turn into a losing proposition today, and a company should be allowed to adapt to new circumstances and be allowed to make new decisions without being anchored and held back by historical decisions (unless under contract).
As fun as it is hold executives to unrealistic standards, they're not fortune-tellers that can predict the future and they're making the best possible decisions they can given the constraints they're under. And I don't begrudge them if those decisions are in their own best interest, such as is their responsibility.
I'll give Docker the benefit of the doubt that this wasn't a bait-and-switch, that they never excepted it to become so successful, and that costs outpaced their ability to monetize the success and were eating into cash reserves faster than plan. I think the current outcome isn't so bad, and that we're still getting a considerable amount of value for free. It's unfortunate that some people are only finding out now, and are now under pressure to address an issue they didn't sign up for.
Standing up your own registry is trivial at the kind of scales (dozens-to-hundreds of images pulls per day!) that we're talking about. It's just expensive, so people want Docker, Inc. to do it for free. Well...
But obviously the real problem is that you're asking the wrong question. We don't "need" a centralized image repository. We WANT one, because the feature that Docker provides that "just use a tarball" doesn't (in addition to general ease-of-use, of course) is authentication, validation and security. And that's valuable, which is why people here are so pissed off that it's being locked behind a paywall.
But given that it has value... sorry folks, someone's got to pay for it. You can duplicate it yourself, but that is obviously an engineering problem with costs.
Just write the check if you're a heavy user. It's an obvious service with an obvious value proposition. It just sucks if it wasn't part of your earlier accounting.