The biggest problem with this idea is that it doesn't really have a good market space. If your company revenue is too small, your subscription backed debt is just an inferior financial product compared to equity, which handles risk much better. If your company revenue is large enough, you have plenty of financial tools to keep your company fiscally healthy. The only time I can see it being useful is if you want to trick rich non-investors to give you money by pretending that your startup has value when it doesn't (similar to an ICO).
Let's say you had a subscription user base and the retention / LTV data to convince finance people to treat it as a security. That usually is the sign of a successful startup, and you'd also likely have access to venture capital as well.
As a founder, is it worth your time to come up with a new financial product and convince people to buy it? In my opinion, you're probably better off doing a round because it will close much quicker and you'll know what to expect, and you can focus on growing your business instead of convincing everyone of your non-standardized, not well understood financial product offering. Good luck closing a group of institutional investors with that.
Now, why do finance people create new financial products? One reason is to investment larger amounts of money all at once - so create asset classes and then buy them in bulk because you have a $5b dollar fund and can only afford to look at $500m deals or more.
This is probably the only reason why you'd want to create a subscription backed debt - collect them up and allow people to participate in returns on startups without becoming a VC. But this is much more likely to be a repeat of the mortgage crisis rather than being actually beneficial to the economy.