I had an idea for the hero subscription. Instead of paying a monthly fee, what if the account tier also had a one time purchase? I would love to use D&D beyond to hold more than 6 characters, but I'd rather just purchase a one time account upgrade. This idea could also work with a master tier. The starting price could be a little bit more than a yearly subscription price.

I'll do my best to explain why I voted "bad idea". It's a bit long - there's a TL;DR at the bottom if you want to skip to it.

The subscription is designed the way it is to ensure continued income even if sales of books decreases. Books and such are single-time purchases and once made you no longer need to ever buy again and a lot of that cost goes back to WotC not D&D beyond to pay for the licenses. Therefore the single-time purchases offer no future security if sales lull or WotC take an extended break from producing new books. During those times D&D Beyond is still incurring new costs for providing the ongoing services and hosting of large volumes of data. The subscriptions contribute to those costs and offers the security they need to ensure things keep running during low-book-sale periods. 

Master Tier subs will be even more difficult because of the fact they allow people to share content to others. A single time fee of providing potentially $600 worth content to 36 people forever is basically going to produce a net loss as the only way they can legally do that would be to provide the user multiple licenses to all the content - which they have to pay for - while getting a negligible amount back from you (I mean, would you upgrade the account for over $21,000? - nope) meaning, every upgrade would result in significant loss. The only reason why you get it now is because of the temporary nature of the access. They are not permitted to anything different - it's WotC who determines what sharing can be offered.

This site offers an advanced character sheet tool, easy reference of all free Basic Rules and EEPC content plus Unearthed Arcana, easy search of Sage Advice, and ensures everything is automatically updated with any released Errata , plus the ability to create all sorts of Homebrew to share with others - all for the price of absolutely nothing. This is, in and of itself, a lot to offer and generates a very large amount of costs - which is only possible mostly because of those of us who do subs. And the sub recurring price ensures that new income is coming in steadily for as long as they do offer these costly services. 

This is why so many companies that offer a content-access based services require recurring subs: future security. It's also a safety net: if they ever stop being successful and go under, people are not complaining about their "lifetime sub" payments. Sure they can have disclaimers, but having worked in customer services I can tell you most people who would complain skip those and will complain anyway. 

TL;DR: one-time life-time purchases are less profitable and offer less future-security for their ongoing services/costs.

You make an incredibly fair point. I guess I was being selfish about the idea. While the idea for a one time account upgrade is a good idea on paper, it has the potential to be a financial nightmare (as you've pointed out) for the great people who run this site.

I think for a one time purchase, it would need to be somewhere between 5x and 10x the yearly cost to make it worth it for DDB.