To better answer that question, we'll take a small look back at our history, and alternatives we could have entertained.
In terms of achieving sustainability by collecting payment from our users directly (instead of say, advertisers), two popular models come to mind:
a. The entire product is behind a paywall (the “Netflix” model)
b. Some, but not all, features are behind a paywall (the “freemium” model)
Ideally for us, the entirety of the product would have been a straight-forward "pay to use" interaction. This would translate to 100% payment-supported interactions between us and our users, in terms of both infrastructure and support. However, early on, launching as a fully paid product can be a bit risky. Users need to trust the product in order to pay for it, yet how can they trust something they haven’t used? A free trial perhaps? Maybe, but with something like notes, it may take several months before you feel settled in. The “some but not all” model seemed to be the best fit for our situation.
Physical circumstances aside, and ideologically, we’re a free product that wants to be paid. Long-term sustainability is hard, and without every user contributing their fair share, longevity enters into question. The freemium model is very conducive towards growth and attracting a large number of free users in hopes they become paid users. However, unchecked exponential growth isn’t what we’re after. If the best of the freemium model is hundreds of millions of free users with a seemingly standard industry conversion rate of 5% to paid, this would call on the 5% to support the entirety of infrastructure and support interactions on behalf of the hundred million. It’s quite an awkward balance.
When we get the reaction that two-factor authentication (or a dark theme, or a Markdown editor) should—nay, must—be a free, core feature, our reaction is: well, who’s paying for it? We certainly can’t be the one to subsidize that feature, and neither can our non-existent advertisers. So we look to our paid Extended subscribers. They have to foot the bill, on behalf of everyone else that may not want to pay at all for any feature of Standard Notes.
And this brings us to the most essential challenge of all: we built Standard Notes to be feature-complete without any extensions. That super slimmed down encrypted notes application where you could only write text and sync it to your other devices with no images or files or formatting? That was the entire vision for Standard Notes. But people wanted more. And we needed a revenue model. So we built an extensions infrastructure that extends functionality without jeopardizing maintainability and stability. By nature, these extensions were “nice-to-have”—that is, a large number of our users said, “I don’t utterly need these features—the free version is sufficient.” And so what happens when a large number of people begin saying “the free version is sufficient”? Well, there become not enough people paying to subsidize free users.
We needed to make Extended more essential. More important. Otherwise, this platform couldn’t exist at all. Some users may think, “this feature should be free” or “that feature should come standard”, and so they imagine a world where the perfect notes app comes fully-suited out of the box at no cost. For us, this world is dangerous, and quite simply, cannot exist.
Some may see our nice free application, and think, this would be near perfect, if only it had a dark theme for free! It’s not too uncommon that a negative review might say that exact thing. Such users may, upon seeing a free application with a nice-to-have feature behind a paywall, think ah, so close! Almost within reach! But the money-minded devs put it behind a paywall! The world thus imagined is that a great piece of software or an instrumental feature should be completely free, because the work to build it is one-time and already done. The world imagined is one where beloved advertisers, who have raised us and shaped our beliefs on the internet since its inception, pay the bill and continue to decide what is and isn’t appropriate content. The world they envision is precisely the one we’re trying to change.
So, the answer to the general question of “why is this feature paid?” is that if it weren’t, we wouldn't be here today, and you wouldn’t be asking that question.